If the domain is connected to another store, then it will need to be removed before it can be added to your current store. If you own the other store, then you'll need to login and remove it. If you don't own that other store, then you'll need to contact Shopify Support to remove it for you.
In any case, as this a account specific issue, contacting Shopify Support will be your best option: https://help.shopify.com/en
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