Required Output Parameter error is issued in the following situation:
(<method name>) method requires an output parameter with (<data type>) data type and Received In property set to 'Body'
You are trying to change the 'Response Format' of a REST API method, or the 'Data Type' or 'Received In' properties of an output parameter in a REST API method.
Follow the instructions in the error message to solve it.
Double-click on the error line to take you directly to the REST API method's 'Response Format' property where the problem was detected.