I try to add an “add-on domain”, but I get an error message saying “Error from domain wrapper: Sorry, you have exceeded the maximum allowed subdomains.” Why?

In cPanel, for every add-on domain that is created, a subdomain is created also.  If the site you are trying to add the add-on domain to has met the quota for subdomains, then you will need to either increase the number of subdomains allowed for that domain or package, or remove an unused subdomain so a new one can be created by cPanel.