The Africa's Talking API will return this response by default whenever we are unable to get a proper response from the Callback URL.
This means that we are getting an erroneous response from your callback URL.(here's an article on what a callback URL is , possible callback responses and how to debug it)
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 support@africastalking.com if you need help.