Have you deleted a domain from a different Unbounce account, and now are trying to add that domain to your current Unbounce Account? This may trigger an additional security measure put into place to prevent domains from being used by anyone other than the domain owner.
The Unbounce Support team can easily fix this issue for you. Contact us by using the Help widget at the bottom left corner of Unbounce. We will verify the ownership of the domain and get it transferred to the Unbounce account of your choice.