my okta appliaction config:
Grant type : Client Credentials, Authorization Code, Refresh Token, Interaction Code
And all rules are passed
And the access testing tool shows that the user can access the application
You could check for the type of authorization server you are using, this failure might mean you might be using Custom authorization server. Confirm this first.
If you are using custom authorization server, check whether you have an access policy rule which allows this test login. For example, check if the app, user, scope combinations allow access.