[dnanian]

The problem is that you didn't fully erase the drive that you had Catalina on, and instead likely either didn't remove one of the two volumes, or left a volume around that was a "partial" volume group (converted by Catalina).

When you restore from Catalina to Mojave, you need to fully erase the drive, recreating a single, non-group, non-Catalina-role volume... I go over this lower in this forum.