Rein here from Shopify. If the Carrier Shipping API did not activate as expected, then you would be best positioned to reach on our to our live support teams through either chat/phone or email. This is because we cannot verify your account ownership through the community forums.
To contact support, you can to log into the help centre using this link here. Once you are logged in, place a search for "Carrier Calculated Shipping" in the search bar and scroll to the bottom of the results page. Once there, click the "Continue" button to move to a live support session with our team.