Hi @Terry Eaton ,
The error means that the application isn't registered, or the wrong application information is included in the registration. Make sure that the application with that identifier is registered under Azure AD > App registrations. (You can do this by pasting the identifier from the error message searching for that identifier there.) Then make sure that the correct Client ID is specified in the application itself (as the Application ID) and matches what you have configured for the registration. Also, please make sure that you code references the correct tenant in the application settings.
In addition, make sure that the app ID/client ID is updated under the Identity in App Service > Your app > Settings > Authentication > Identity Provider > +Add provider
Let me know if this helps and if you still face this issue.
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