What's the exact error message you're getting? If you want to obfuscate the domain name, go ahead.
For example, here's the error message that we get when we try to add a subdomain for a domain that hasn't been added as a domain yet:
Can't add domain: you don't have permission to add to godaddydomain.tld.
I suppose there's also the possibility that the domain was already added to some other account. I don't think I've ever personally seen that error message, but it might be possible that you're seeing that error message.
I recommend submitting a support ticket about your issue if you haven't already. Make sure to provide as many details about your problem as you can. It'll help the guys in support out and make it easier and faster for them to help you!
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