Overview
This error usually occurs when you are not specifying the correct carrier increment, via the API.
To check this, go to your carrier settings in Shiptheory:
The increment will be the number in the URL, for example: https://helm.shiptheory.com/royalmailoba/edit/1
In the above example, "1" is the increment number.
This number increases every time a carrier is added. So if you removed and re-added your carrier, the increment number would be 2, and you would need to update the API call made to Shiptheory to point to the now current increment of 2.
If you encounter any issues, please feel free to contact our support team. You can call us on 0117 403 4313 (UK) / +1 (629) 666-6726 (US) or reach us online here.