You are not allowed to add more than 0 addon domains. The domain was not setup.
You might receive the following error message when trying to add a domain name to your cPanel shared hosting account:
You are not allowed to add more than 0 addon domains. The domain was not setup.
Explanation
This error occurs when you try to add a domain name as an "Addon Domain," which functions as a separate website from your primary domain name, but have an Economy-tier hosting account, which only lets you host one website on it.
Solution
To fix this issue and host multiple websites on your account, you can upgrade your account to a Deluxe, Premium, Ultimate, or Unlimited plan.
If you do not want the domain name you're using to have its own website, you might instead want to add it as an "Alias domain," which will make it load your hosting account's primary domain name's website.