Hi there,
Thank you for reaching out to us!
We are trying to reply and provide some information to older posts, in case others encounter the same situation and it could help them too.
Looking at the settings, I believe everything should be correct. My suspicion is that the current M2M application that you are using was previously set up as a different type, a Native or Spa app for example. Changing the Application type can sometimes cause these issues, despite setting everything correctly. Please see more information on our documentation on this error message : Calling /oauth/token Returns Grant Type 'client_credentials' Not Allowed for the Client
I would advise to create a new M2M application altogether and check with that is you are getting the same error. This should clear out any conflicts that happen to still be ongoing, but not detectable.
Hope that helped!
Gerald