To install ncurses, try sudo apt install ncurses

For the -dev package, try sudo apt install libncurses-dev

There are no -dev packages for libncurses6 nor for libncursesw6.

You need to use the "old-releases" archive for Natty Narwhal; replace archive.linux.duke.edu with old-releases.ubuntu.com in your sources.list file, then run sudo apt-get update, and you should be able to install the packages.

As Huygens suggests though, you should really upgrade to a supported version of Ubuntu.

Certainly solved and not really related to your initial problem but might help someone :)

Check that you have libncurses5-dev package installed

sudo aptitude search %p libncurses

This should give you something like:

i libncurses5 - Bibliothèques partagées pour l'utilisation d'un terminal
i libncurses5-dev - developer's libraries for ncurses

which here means that the package is installed

Same result with:

find /usr -name "libncurse*"

If not:

sudo apt-get install libncurses5-dev

./configure --with-features=huge --with-tlib=ncurses

I figured this out for Ubuntu 24.x though I could not find the exact answer anywhere, so posting this here for posterity:

First, if you are on amd64 make sure you have the i386 foreign architecture configured. Then follow the advice on the download page for curses by modifying your /etc/apt/sources.list.d/ubuntu.sources. Ensure you have a deb entry for http://security.ubuntu.com/ubuntu. In that entry, ensure you have 'focal-security' as an item for 'Suites' and 'universe' as an item for 'Components.' Run "sudo apt update." Then you should be able to install libncurses5:i386.

You should have run (as root)

aptitude update
aptitude search libncurses

that would suggest you the right package name.

Then try

aptitude install libncurses-dev

and

aptitude install kernel-package
aptitude build-dep linux-image linux-image-amd64 

At last, use make-kpkg --initrd binary to compile your kernel. (it will produce *.deb files in the parent directory). You want to configure /etc/kernel-pkg.conf and perhaps /etc/kernel-img.conf

ncurses provides multiple ABIs in the same distribution. Previously, the default ABI was 5, and now it's 6. libncurses5 provides the v5 ABI, and libncurses6 provides the v6 ABI. You cannot use a piece of software linked against a shared library with an incompatible ABI like this because the shared library paths differ across ABIs so people don't accidentally load the wrong one and break their programs.

If you want to run an existing program linked against ABI v5, then you need the libncurses5 package. You do not need the -dev package unless you are developing against that package. If you are recompiling an existing package, you should use libncurses-dev, which will build against the latest version of ncurses unless you are sure you need to build against the old version. That package will include the curses.h header.

If you want to specify the location of the curses header, you should do that wit angle brackets: #include <curses.h>. It is a system path, and it is definitely provided by libncurses-dev.

If your program is using the standard curses API that's in common use, then recompiling it for a new version of ncurses should not change anything. The ABI changes likely consisted of changing some internal structures which do not affect the display but may enable new terminal features. The behavior of the curses API is standardized and breaking changes would not be expected.

It's likely that your TERM setting is indeed the problem. If you're running in an SSH session on another computer, it's possible that the system you're running it on lacks that terminal definition as part of the standard set, and requires an additional package (on Debian and Ubuntu, ncurses-term) that includes more terminal definitions. You can try running infocmp xterm-256color on that system, assuming it's running ncurses, to see if you do have that terminal definition. It will print terminal info if you do, and an error if you don't.

If you're in a screen or tmux session, then xterm-256color would be the wrong terminal type, and screen-256color would be a better choice. Even if not, you may want to check your terminal settings for the type it's emulating, since some terminal emulators have support for multiple terminal types.

The package libncurses5-dev is available on base:main repository for Ubuntu 14.04.

So, make sure you have enable main & universe repositories:

The main component contains applications that are free software, can be freely redistributed and are fully supported by the Ubuntu team. & The universe component is a snapshot of the free, open-source, and Linux world.

If you choose command-line, following command may help you:

sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) main universe"
sudo apt-get update
sudo apt-get install libncurses5-dev

If you face any unmet-dependencies then refer How do I resolve unmet dependencies after adding a PPA?

Did you try what was suggested? i.e. libncurses-dev or ncurses-devel ? Not libncurses5-dev

Which distro/version are you using?

Looks like libncurses-dev is a virtual package which brings up libncurses5-dev in Ubuntu 16.04. Fedora 28 has ncurses-devel available in the updates repo.