Change the user to postgres :

su - postgres

Create User for Postgres (in the shell and NOT with psql)

$ createuser testuser

Create Database (same)

$ createdb testdb

Acces the postgres Shell

psql ( enter the password for postgressql)

Provide the privileges to the postgres user

$ alter user testuser with encrypted password 'qwerty';
$ grant all privileges on database testdb to testuser;

There are two different ways of creating a database in PostgreSQL. One is from the command line. The other is from the PostgreSQL console.

Command Line

In order to be able to create a database from the command line, you must first change to a user with rights to create a database. As you have shown with the \du command above, the user with those rights is postgres.

root@eric-desktop:/home/eric# su postgres
postgres@eric-desktop:/home/eric$ createdb prismatest2
postgres@eric-desktop:/home/eric$

As you can see, the creatdb command does not give any feedback. You need to confirm the creation by listing the current databases:

psql -U postgres -l
                                                 List of databases
    Name     |  Owner   | Encoding |   Collate   |    Ctype    | ICU Locale | Locale Provider |   Access privileges   

-------------+----------+----------+-------------+-------------+------------+-----------------+-----------------------

 postgres    | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest2 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

PostgreSQL Console

Another possibility is to use the PostgreSQL console. To enter the console:

root@eric-desktop:/home/eric# sudo -u postgres psql psql (15.7 (Debian 15.7-0+deb12u1)) Type "help" for help.

postgres=# 
postgres=# CREATE DATABASE "prismatest3";
CREATE DATABASE
postgres=# 

To confirm the creation of the database, list the databases:

postgres=# \l

                                                 List of databases
    Name     |  Owner   | Encoding |   Collate   |    Ctype    | ICU Locale | Locale Provider |   Access privileges   

-------------+----------+----------+-------------+-------------+------------+-----------------+-----------------------

 postgres    | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest2 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            | 

 prismatest3 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |            | libc            |