Can't finish Catalina install, restart loop. Help!

I downloaded the Catalina software and restarted my computer to finish installing it. But halfway through the install, I get an error message saying, "macOS could not be installed on your computer, A disk with a mount point is required. : (-69854) Quit the installer to restart your computer and try again."

When I restart it, I get the same error message. I tried selecting a different startup disk, but there is no other option (the box is empty/blank). How do I get out of this loop?!