Faceing same issue here.

https://developer.apple.com/forums/thread/675742 looks like this problem is not happen at first time.

This is my url: https://appleid.apple.com/auth/authorize?client_id=com.service.paxton.stockApp&redirect_uri=https%3A%2F%2Ftrusting-tick-trivially.ngrok-free.app%2Fauth%2Fapple%2Fcallback&scope=name%20email&response_type=code&response_mode=form_post&state=i8WcgRofXdtzH17CdyJ9HYAqJdpOvrS7X6J3Mdnb&nonce=c6019718-4c4b-41c8-8a97-ad5dbd03b6e7.i8WcgRofXdtzH17CdyJ9HYAqJdpOvrS7X6J3Mdnb looks like we're doing the same thing, but I'm still getting the invalid_client error today.

I posted an article yesterday describing my settings, but I still can’t figure out what I did wrong.

Update, here is my configration screenshot.