I got it! I discorvered what was my problem.

I've uninstalled all the 2015-2019 versions, allthough i found a 2019 version installed that i had not found before and uninstalled it. Then i run again the setup and i get it, finally!

Note: i had to copy the "prereq" files from the installer folder to: C:Program FilesANSYS IncANSYS Studentv201prereq