What can I do if I want to park a domain (add an alias) in cPanel and get the "You may not have more than 0 of the resource parked domains" error?
This setting can be modified via the "Modify an Account" section of WHM. Navigate to (WHM >> Account Functions >> Modify an Account) and then select the domain in question from the list.
From here, you can find the "Max Parked Domains" section and increase this directive to the desired number. You can also set it to unlimited. After this, you can save at the bottom.
For more information regarding "Modify an Account" and "Max Parked Domains", please see the following documentation.