Azure AD Graph API - AADSTS70001: Application with identifier xxx was not found
If you happen to encounter the error mentioned in the title of this post which is something that bugged me for a while...it might be due to a wrong sequence of actions. As you might know, when creating a custom application that needs to access an Azure Active Directory with the Graph API, you need to grant permissions to the App, in the configuration page of the App or through PowerShell.
Make sure you grant the permissions BEFORE you create the Client Secret (keys) because otherwise, your key won't contain the security information and you'll most likely fall into the problem mentioned in the subject of this post. As the error message is a bit misleading since you'd expect more something related to security, you could lose time in troubleshooting.
So pay attention to perform the operations in the right order. The keys hold the security information. Similarly, if you revoke some security settings at Application level, it won't affect the previously created keys so make sure to revoke them as well if necessary.