After following Sign in your user with Credential Manager, I adapted it to my use case and backend.
Creating a passkey worked fine with Samsung Pass and Dashlane, but not with Google Password Manager that raised an unhelpful androidx.credentials.exceptions.CreateCredentialCustomException: 34000: null.
After a while of trial and error, I realized that my backend's PublicKeyCredentialCreationOptions was lacking an authenticatorSelection claim, unlike the example in the Android guide.
This claim is not marked as required in WebAuthn 2 spec. Still, the presence of a non-empty authenticatorSelection claim is what makes passkey creation with Google Password Manager work. I also isolated this behaviour using the example app from android repos.
I couldn't find documentation on this requirement. Is this just bad spec implementation from Google Password Manager? Did I misunderstand the spec? Did I miss a piece of documentation ?