Which version of cmake do you have?
cmake --version
If it’s less than 3.15 then you need to install a later version. Instructions here: https://forum.audacityteam.org/t/building-audacity-2-4-2-on-debian-10-with-cmake-3-18/58352/1
Once you have a recent enough version of cmake, I think these instructions should work: https://forum.audacityteam.org/t/building-audacity-2-4-2-on-ubuntu-20-04/57719/1
(I’m also using Xubuntu 18.04)