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Don't use the outdated Elgg 2.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 ;)
info@elgg.org
Security issues should be reported to security@elgg.org!
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