Possible cause.

A. You already have added it and it is active on your account.

(What do I Do?? There is No need to add it again, do nothing.)

B. You already have added it and it is NOT currently active on your account.

(What do I Do?? It was Deleted by you or suspended by the AdHitz Staff, Open a Support Ticket to see if you can get it restored.)

C. You have Never added it to your account before, But it is still on another user's account see case A or case B as described above.

(What do I Do?? If you are the new owner of the site but you can't add it, Open a Ticket and tell the AdHitz staff you have purchased the domain and are the new owner but can't add it to your account. Ask that they approve a domain transfer so you can add it.)