official MIUI 12 beta is rolling out, it's stable candidate and if everything works well we probably can expect stable .eu ROM, I guess .eu stable is not released until there's official Xiaomi stable of MIUI 12, correct me if I'm wrong
go to Mount in TWRP and make sure system is checked. to me it wasn't checked. now when u check System under mount -> reboot to recovery -> and then install Magisk again. it worked for me.
anyone having problem with rooting (Magisk 20.4) - Mi 9 Lite?
After dirty flashing newest ROM, when I try to install Magisk in TWRP it says error - cannot mount system/