No, Eu Rom you can flash via your phone recovery (or Fastboot way with computer connected).
For MiFlash installation you need to download appropriate Ursa Rom where file suffix is '.tgz' !!!
You can find it on the official MIUI forum download page.
When you open TWRP recovery first time you must approve the recovery can change your system.
If you won’t do it TWRP can’t format your be able to format your system.
See instruction in movie:
in time ~1.20 point 5.
Good luck.
+
I got the same during EU ROM installation on my Mi8EE.
Solution was:
Reboot into TWRP and do a FORMAT DATA to clean all on the phone.
Restart into TWRP recovery again.
Connect to the PC and upload appropriate ROM.
Install... Reboot... Done... :)))
Good Luck.
Thank you for your reply.
Why did you change the installer from URSA to Dipper?
Did your installation/update was done on Mi8 standard version, not the EE one?