Dear Enrico,
the instructions for installing FLUKA on WSL are a bit outdated at the moment. You don’t need to use Ubuntu 18.04, but you can use the newer versions as well (with the gfor9 version of FLUKA).
Currently there is an issue with the 18.04 packages of Flair (See: FLUKA 4-3.0 for ubuntu 18.04 - #4 by dikontog). Once it is fixed, the setup script will be updated to allow installations with Ubuntu 18.04 again.
Installing on Ubuntu 20.04 or 22.04 should work.
Cheers,
David