If you bought a used smart doorbell or smart camera and try to pair it with your smart app, you may get the message that the device is already paired. This is because the device has probably been paired with another user before.
If the device has already been paired with a user, pairing with other accounts is blocked to protect the privacy and security of the original owner.
To still be able to use the device, you must unlink the device from the original account. This can sometimes be tricky, especially if you don't know the previous owner or don't have access to the account.