Hi,
not sure what 20.04 version you are looking for.
Looking at Install package graphics:darktable / darktable 
OBS provides in the “Latest Release” section darktable 3.0.2 package for xUbuntu 20.04 : https://download.opensuse.org/repositories/graphics:/darktable/xUbuntu_20.04/amd64/darktable_3.0.2-1.1_amd64.deb