Nala isn't officially supported on Ubuntu earlier than version 22.04.
Nala is available in the Universe repository as a backport for Ubuntu 22.04, and as a "normal" release on Ubuntu 22.10 and newer.
It's also available in the Volian Scar repository. The installation Wiki page here clearly states:
Ubuntu 18.04 / Debian Buster and older
Unfortunately Nala will not work on these releases or older.
The python versions are too old, and they don't have the required packages.
Having said that you may be able to look further below and install from source. These distros will
not be officially supported.
Nala was previously available for Ubuntu 20.04 under the package name nala-legacy, but the developers have dropped providing binary packages for this branch, since it was too difficult to maintain.
So the only option to get Nala working on older systems is to manually compile from source, and in addition fix all required Python dependencies (which may be a complicated task).