You must install the Linux build support package. You may do this using Unity Hub, if you use it.
Viewing post in How to compiled unity for Linux. (With images)
hey buddy try out my last version of the game ;) I´m actually having problmes with the linux ! it crashes in the end I´ll try to fix that and have a linux build up and running the game is way better now with the melee and ranged combat and there are more enemies and areas I think you´re gonna like it ! cheers