That error message looks like a search command error. Do you have command=register in your url? The register request example was using 'search', but it has been changed to 'register'.
Also it's possible that the IP address your script is connecting from is not correct (set in your Dynadot account: https://www.dynadot.com/account/domain/setting/api.html)
The system may respond with a message like that but you need to check the error details...
domain registration api problem