What to do if Sigfox backend CRA rejects the DevKit registration?
You are most likely missing the PAC code or entering the wrong one.
There are two steps to take:
1) Go to the Devices tab in your Sigfox backend and check deleted devices. Search for the ID you're trying to register and if it is there, check the PAC code (often it will be different from the one you received when the devices were new).
2) If you cannot access the deleted devices tab or the PAC code that's in there doesn't work as well, contact the support desk of your local Sigfox Operator.