The Africa's Talking API will return this response by default whenever we are unable to get a proper response from the Callback URL.
You can view the error message in your sessions logs. By following the below steps.
Go to USSD sessions
Select the three dots below the word actions
Check what app response or error message we are getting from your callback URL. That should help debug your code.
It might be anything from Status 500 to Not starting your response with 'CON' or 'END' .
Email firstname.lastname@example.org if you need help.