django-mssql-backend currently supporting till Django 3.0.10. The temporary fix is to downgrade your Django version from Django 3.1.x to 3.0.10.

bug request raised here: https://github.com/ESSolutions/django-mssql-backend/issues/63

Edited:

Another solution, use Microsoft’s official Django support package to overcome all the issues, the support is very good.

https://github.com/microsoft/mssql-django

I ran in to this same issue today. Roll back to Django 3.0.x (I went back to 3.0.6) and then run your migrations.

I realize that’s not a fix, but at least it’s a work around to get you going. Once you migrate, you should be able to update back to 3.1 and it will run fine.