"unable to mount /system" in TWRP

Hello, I was trying to install ROM Arcatarc's EMUmIZERO SUPERLEGGERA. I installed TWRP v2.8.2.0, I did backup of all my data and system, wiped it and tried to install that ROM, but always got message "unable to mount /system". So I tried to restore my backup, but always ended with same message "unable to mount /system". I got this message when I do pretty much everything. Now I have no system and cannot use my phone. Please help.