Good day Scott,
Before this, I never had issue of resolution to Ubuntu guest. I am not sure why there is now problem (having resolution problem to Ubuntu guest, since I started to use Virtual Box 6.1.
I now am not sure if Virtual Box addition has been applied properly to the current Ubuntu guest, or not.
The manual link above shows why Virtualbox and Guest Additions need things done to the OS before installation. If you haven't handled the prerequisites in the past, it could be that the OS had the items in place at that time. But the error message says the prerequisites are not in place now, so they have to be done again before installing Guest Additions.