I think it is a bug. ModelSim cannot be called from Quartus on Ubuntu
1) I have Quartus Prime Lite 21.1 installed on Ubuntu 20.04 and it is working fine.
2) I also have ModelSim 20.1.1 and it is working fine
3) I've put the right path to ModelSim on Quartus Prime, ...
Nop. What I did instead was step back the Quartus and the ModelSim to version 20.1, so now I can use it normally. But I could not make the previous arrangement to work.