This error is commonly seen when first setting up Magento 2 with Shiptheory. It can be resolved by first inputting the API key from Shiptheory into Magento 2 Shiptheory Configuration page, but making sure that the setting to "Enable Webhooks" is set to No:

Then hit "Save Config":

Then change the setting from no to yes and hit "Save Config" again: 

This should resolve the issue and let you go ahead with using Magento 2 with Shiptheory.

If you are stuck or still having issues, please get in contact with Shiptheory support.