First of all, amazing work, Rom is very stable, fully loaded with all necessary functionalities and probably the best Rom. I've been a long time user of Evolution 2X. It has always been my daily driver until my switch to Android 10. Once agai...
Try using the older version of the mi unlock tool. Probably some 2.2 or 2.3 version.
In case it doesn't work, then sign-out from the mi account in phone, then create a new account, sign-in. Don't apply for unlock permission in the Developer...