Hello,
Sorry for the problem you're having with adding the domain. If you're trying to create a separate website so that the domain is its own account under your reseller account, then you need to create a new cPanel account. If you're trying to add it to an existing account, then you need to add-an Add-on domain through cPanel. If you have any further questions in regards to your Reseller account, please make sure to review our Everything Reseller Education Channel.
If you have any further questions or comments, please let us know.
Regards,
Arnel C.