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/
Then cd to your project folder and run the Core ML compiler:
xrun coremlc compile YourModel.mlmodel .
This creates a YourModel.mlmodelc folder. Add this folder to your Xcode project.
In Xcode 11, open YourModel.mlmodel and view the auto-generated source code (the mlmodel needs to be part of a project for this). Copy-paste this source code into a new source file, YourModel.swift. Add this source file to your Xcode project too.
Remove YourModel.mlmodel from the project (just the reference, don't delete the file).
Open your project in Xcode 12. Disable automatic code generation for Core ML models.
To load your model, do the following:
let config = MLModelConfiguration()
let url = Bundle.main.url(forResource: "YourModel", withExtension: "mlmodelc")!
let model = try YourModel(contentsOf: url, configuration: config)
It is possible your app will crash when loading the model. If that happens, add the line config.computeUnits = .cpuOnly and try again.