The same thing happened to me. I had already implemented Apple Sign In for one web application, and then I needed to implement it for another one. I'm using the popup method to get the authorization code from Apple. I double-checked the Service ID and return URLs I'm using, but I'm still getting the invalid_client error on the first request made to Apple to retrieve the authorization code. I'm implementing it the same way I did before. I can't see where is the problem