Thank you for getting in touch about this. Whilst I cannot say for sure that this is what is happening here, this issue can sometimes occur when the domain in question was previously owned by someone else and is still attached to another Shopify store, which disallows you from adding it to your own store. I can investigate this for you and should be able to help, but first will need to authenticate you on your store, so I will send you an email shortly to discuss this with you in more detail. Please reply to my email and we can proceed from there.
Victor | Shopify Social Care