When signing up for a Debutify account, you are asked to provide a Shopify URL. That's then linked to your account.
If you get an error notification that the Shopify store domain is already registered, please request a Live Chat support agent to assist.
After creating a Debutify account, you may want to change the Shopify URL registered in Debutify. Contact a Live Chat support agent for assistance. They will need the current and new Shopify URL to effect the change and they'll send confirmation via email.