When adding an addon domain to a cPanel account, the following error occurs where X is the addon domain limit in the accounts package and domain.tld is the domain name:
Your addon domain limit of X addon domains has been reached. The addon domain domain.tld was not added.
This error is returned when an account has reached the addon domain limit configured in the accounts package.
Increase the value of "Max Addon Domains" in the package assigned to the cPanel account or remove addon domains that are not in use via cPanel >> Addon Domains.
Please see the following article for instructions on how to edit a package: