I ran into a problem with Xcode 12's Core ML compiler, and several people have emailed me about what appears to be the same issue.
The workaround is to use Xcode 11 to compile the Core ML model. This means you still need to have Xcode 11 installed (which is a good idea anyway).
What worked for me is the following...
From the Terminal, switch to the Xcode 11.7 installation (your path may be different):
sudo xcode-select -s /Applications/Xcode11.7.app/Contents/Developer/