Accidently installed Catalina I did not have a time machine back up and want to go back to High Sierra 10.13
Manually copy the files over from Time Machine.

Just because you can’t restore doesn’t mean you can’t browse and copy.  Understand, that you’re not going to restore applications and settings, but you’ll be able to bring over your data with no problem.

In fact, I’m doing this exact process with my new install of Catalina rather doing a restoration or using Migration Assistant.