I can help you kindly contact firstname.lastname@example.org
Ash here from the Shopify team. Thank you for posting your question in the Shopify Community Forums!
It sounds like your custom domain is currently associated with another Shopify account, preventing you from adding it to the store you're currently working on. If you don't own the other Shopify store, I can help out further here. I've just sent an email to the email address associated with your Shopify Community account to authenticate you. Please reply to that email and we can proceed with the next steps together.