Azure App Registrations can only be deleted if they aren’t shared.
Some app registrations have a settings screen that lets you change the shared flag, while some do not.
For those that don’t the fix is fairly easy:
Go to Azure AD in portal.azure.com, find the App registration you’re having trouble deleting.
- Click the manifest button
- Change “availableToOtherTenants” to false
- Click Save
Now you’ll see the ‘delete’ icon will ‘light up’ (turns from gray to black, indicating that you can use it now.
Hope this helps!