apt can’t locate openjdk-16-jre-headless because the OpenJDK 16 packages aren’t available in any Debian release.

The only release available to you in Debian 10 is OpenJDK 11. Debian 11 has OpenJDK 17. Both of these are releases with long-term support.

UPDATE: installation without root privileges below

I advise you to not install packages manually on ubuntu system if there is already a (semi-official) repository able to solve your problem. Further, use Oracle JDK for development, just to avoid (very sporadic) compatibility issues (i've tried many years ago, it's surely better now).

Add the webupd8 repo to your system:

Copysudo add-apt-repository ppa:webupd8team/java
sudo apt-get update

Install your preferred version of jdk (versions from java-6 to java-9 available):

Copysudo apt-get install oracle-java8-installer

You can also install multiple version of jdk, mixing openjdk and oracle versions. Then you can use the command update-java-alternatives to switch between installed version:

Copy# list available jdk
update-java-alternatives --list

# use jdk7
sudo update-java-alternatives --set java-7-oracle

# use jdk8
sudo update-java-alternatives --set java-8-oracle

Requirements

If you get add-apt-repository: command not found be sure to have software-properties-common installed:

Copysudo apt-get install software-properties-common

If you're using an older version Ubuntu:

Copysudo apt-get install python-software-properties

JDK installation without root privileges

If you haven't administrator rights on your target machine your simplest bet is to use sdkman to install the zulu certified openjdk:

Copycurl -s "https://get.sdkman.io" | bash
source "$HOME/.sdkman/bin/sdkman-init.sh"
sdk install java

NOTE: sdkman allow to install also the official Oracle JDK, although it's not a the default option. View available versions with:

Copysdk ls java

Install the chosen version with:

Copysdk install java <version>

For example:

Copysdk install java 9.0.1-oracle

Glossary of commands

• sudo <command> [command_arguments]: execute a command with the superuser privilege.

• add-apt-repository <PPA_id>: Ubuntu (just like every Debian derivatives and generally speaking every Linux distribution) has a main repository of packages that handle things like package dependencies and updating. In Ubuntu is possible to extend the main repository using a PPA (Personal Package Archive) that usually contains packages not available in the system (just like oracle jdk) or updated versions of available ones (example: LibreOffice 5 in LTS is available only through this PPA).

• apt-get [install|update|upgrade|purge|...]: it's "the" command-line package handler used to manipulate the state of every repository on the system (installing / updating / upgrading can be viewed as an alteration of the repository current state).

In our case: with the command sudo add-apt-repository ppa:webupd8team/java we inform the system that the next repository update must retrieve packages information also from webupd8 repo.

With sudo apt-get update we actually update the system repository (all this operations requires superuser privileges, so we prepend sudo to the commands).

sudo apt-get install oracle-java8-installer

• update-java-alternatives (a specific java version of update-alternatives): in Ubuntu several packages provides the same functionality (browse the internet, compile mails, edit a text file or provides java/javac executables...). To allows the system to choose the user favourites tool given a specific task a mechanism using symlinks under /etc/alternatives/ is used. Try to update the jdk as indicated above (switch between java 7 and java 8) and view how change the output of this command:

  ls -l /etc/alternatives/java*

In our case: sudo update-java-alternatives --set java-8-oracle update symlinks under /etc/alternatives to point to java-8-oracle executables.

Extras:

• man <command>: start using man to read a really well written and detailed help on (almost) every shell command and its options (every command i mention in this little answer has a man page, try man update-java-alternatives).

• apt-cache search <search_key>: query the APT cache to search for a package related with the search_key provided (can be the package name or some word in package description).

• apt-cache show <package>: provides APT information for a specific package (package version, installed or not, description).

Try a quick apt-get update to get the latest package lists from repos, and then apt-cache search openjdk to check out what the latest offered version is that's available to you, if any are.

Most likely if this is a fresh base install you're going to have to go into your /etc/apt/sources.list and make sure you have the non-free package lists enabled, which aren't enabled by default in Debian I believe. So check this How do I enable non-free packages on Debian?

You have two alternatives:

1. Manually download the .tar.gz file for Oracle Java SE Development Kit 8 from the official Oracle website, and compile it by following the instructions in How do I install a .tar.gz (or .tar.bz2) file?.

2. Install Java 7 from the default Debian jessie repositories.

   sudo apt-get install openjdk-7-jdk
   

The default Java JDK package is openjdk-8-jdk in Debian stretch and openjdk-11-jdk in buster.

Oracle is no longer maintaining OpenJDK 8 and OpenJDK 11. Red Hat is providing bug fixes and security patches for OpenJDK 8 and OpenJDK 11, taking over from Oracle. If you are using OpenJDK from the default Debian repositories, Oracle's license cancellation doesn't matter at all because Debian will continue to support their Java packages the same way they support every other package in their repositories.

This article you followed is outdated as the "PPA" it tries to add and use does not exist anymore. Worry not, I believe you are better off without Oracle's package anyway.

So, how do you get Java?
Simply install one of the so called openjdk packages.
You can get a list of them by using the command: apt-cache search openjdk and then using sudo apt-get install PACKAGENAME.
I believe you are looking for openjdk-8-jdk, which can be installed by using:
sudo apt-get install openjdk-8-jdk.

These packages work just as well as the Oracle one did/does.

Here is an online list of these packages:
https://packages.debian.org/search?keywords=openjdk&searchon=names&suite=stable&section=all

[17-08-2022] thank to @nobody (see comment to the question)

For all older version use this link:

https://launchpad.net/~openjdk-r/+archive/ubuntu/ppa

sudo add-apt-repository ppa:openjdk-r/ppa
sudo apt update

and select your version:

sudo apt install openjdk-15-jdk

There must be some lines before E: Unable to parse package file showing errors like this:

E: Unable to parse package file /var/lib/apt/lists/…

You need to

sudo rm -i /var/lib/apt/lists/*
sudo apt update

Do not forget the asterisk * in the first command!

Command Line option - Ubuntu

Copysudo apt-get install python-software-properties
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update

Then in terminal

Copysudo apt-get install oracle-java8-installer

When there are multiple Java installations on your System, the Java version to use as default can be chosen. To do this, execute the following command.

Copysudo update-alternatives --config java
sudo update-alternatives --config javac
sudo update-alternatives --config javaws

Edit - Manual Java Installation

Download oracle jdk

Copyhttp://www.oracle.com/technetwork/java/javase/downloads/index.html

Extract zip into desired folder

Copy e.g  /usr/local/  after extract /usr/local/jdk1.8.0_65

Setup

Copysudo update-alternatives --install  /usr/bin/java java /usr/local/jdk1.8.0_65/bin/java 1
sudo update-alternatives --install  /usr/bin/javac javac /usr/local/jdk1.8.0_65/bin/javac 1
sudo update-alternatives --install  /usr/bin/javaws javaws /usr/local/jdk1.8.0_65/bin/javaws 1

sudo update-alternatives --set  java /usr/local/jdk1.8.0_65/bin/java
sudo update-alternatives --set  javac /usr/local/jdk1.8.0_65/bin/javac
sudo update-alternatives --set  javaws /usr/local/jdk1.8.0_65/bin/javaws

Edit /etc/environment set JAVA_HOME path for external applications like Eclipse and Idea

Step 1: Install Java 8 (JDK 8)

Add the webupd8team Java PPA repository in our system and install Oracle java8 using following set of commands.

$ sudo add-apt-repository ppa:webupd8team/java
$ sudo apt-get update
$ sudo apt-get install oracle-java8-installer

Step 2: Verify JAVA Version

After successfully installing Oracle Java using above step verify installed version using following command.

$ java -version
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

Step 3: Setup JAVA Environment

Webupd8team is providing a package to set environment variables, install this package using following command.

$ sudo apt-get install oracle-java8-set-default

1. add the following line to /etc/apt/sources.list

   deb http://deb.debian.org/debian/ sid main

2. install openjdk8

   //Update the repositories<br/>
   $ sudo apt-get update<br/>
   $ sudo apt-get install -y openjdk-8-jdk
   

In fact, I think it is better to search for your question first before ask.

How to install openjdk-8-jdk on Debian 10 (Buster)?