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.

You may also see a similar error here;

"Something went wrong while saving this configuration: Shiptheory setup API returned an incorrect http code (307) "

This error is received due to the Shiptheory extension in your Magento 2 store being outdated. To resolve this, you'll want to ensure that you update the extension and ensure that the old one is removed, you should then not see this error when going through setup. Please see how to get the up to date extension here: Magento 2 Install

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