Restore my iMac from Time Machine Backup?

Boot into the Recovery volume (boot with the Command + R keys held down), select Disk Utility and erase the entire drive. Then reinstall the system. In the first boot don't create a user account but select to migrate from your Time Machine backup made just prior to your Catalina upgrade.

try contact Apple Support, print screen step by step what you did and show them.

They are very patient and helpful and managed to solved my problem

thanks...i also managed to install back Mojave using Time Machine after Apple Support Technicians guided me step by step how to it.

I had the same message and can't restore from Time Machine.

Have you manage to solve the problem and restore from Time Machine?

If yes please advise how.

Still hasn’t worked for me.... tbh I’m at a loss... the disk is erased and the backup won’t restore now. I have no clue what else to do and I’m really freaked out.

yes

And it won't install. What year is your Mac? Also what type of drive do you have in it: rotational HD, SSD or Fusion?

When you erased the drive what format did you set it to?

Did you boot into the Recovery volume and restore the system from there?