I have the same problem.
I've double-checked multiple times:
The App Identifier
The App Identifier has the Sign In with Apple capability enabled
The Service Identifier
The Service Identifier is linked to the correct Primary App ID and Website URLs
The "Configure Sign In with Apple for Email Communication" setting has also passed SPF verification
It took me almost two days to debug, but I got nothing. I referenced all of Apple’s API documentation and still couldn’t find the issue. I even tried the official Sign in with Apple JS with a minimal viable example — but that failed too.
I believe Apple only verifies the client_id and redirect_uri, since the other parameters are the same as in working examples.
For example, Dropbox uses this URL:
https://appleid.apple.com/auth/authorize?client_id=com.dropbox.Backend&redirect_uri=https%3A%2F%2Fwww.dropbox.com%2Fapple%2Fauthcallback&response_mode=form_post&response_type=code&scope=email%20name&state=abc
client_id=com.dropbox.Backend
redirect_uri=https%3A%2F%2Fwww.dropbox.com%2Fapple%2Fauthcallback
response_mode=form_post
response_type=code
scope=email%20name
state=abc