Sometimes while adding an Addon domain you might have seen an error- You are using your maximum allotment of Addon domain.This issue only appears when you have limited Addon domains in your hosting plan.
So if you are a shared hosting user, you just need to contact your hosting service provider to exceed your limit and revise your hosting plan.
However if you are a Root or a Reseller, then in that case you can easily fix this issue.
Step::1 Login to your WHM Root/Reseller interface
Step::2 Jump on to the Account Information section
Step::3 Then click on List Accounts
Step::4 Here you will see the list of domains under your server, simply click on the + icon infront of the domain for which you are facing the problem.
Step::5 Now click over Modify Account
Step::6 Now scroll down to the Addon Domains section and increase the limit
Step::7 Once you are done with the changes, simply click over Save button