There is no difficulty to install Elgg on this Ubuntu version.

For latest Elgg 4 you should install PHP 7.4+

Docs: http://learn.elgg.org/en/stable/intro/install.html

For latest Elgg 3 you can still use PHP 7.2+

Docs: http://learn.elgg.org/en/3.3/intro/install.html

Don't use the outdated Elgg 2.

You can read also this tutorial; just don't forget about the published date ;)