ResponseCode 401 is treated as error

Tutorials: https://developers.sap.com/tutorials/cp-starter-isuite-api-management.html
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I am following the above tutorial. When I test the API, it is sending me the following error:

{"fault":{"faultstring":"Execution of ServiceCallout getoauthtoken failed. Reason: ResponseCode 401 is treated as error","detail":{"errorcode":"steps.servicecallout.ExecutionFailed"}}}

Can you advise where the problem could be? The CPI seems to be a total blackbox compared to the classic PI.

Thanks Jonathan.