There are two common problems when setting a new addon domain:

1. You are trying to set a Parked domain as an Addon domain. In order to resolve this please remove first the domain in question from the Parked domains and then set it in the Addon domains;
2. You cannot activate new addon domains, as you hosting type does not allow this. In SiteGround New addons can be added only to higher hosting plans like Hosting PLUS, Semi dedicated hosting etc. In order to upgrade a standard shared hosting account so that you can use more addon domains please go to the order section of your Customer's Area.

If neither of the above helps, please contact our support team by posting a ticket in the cPanel category.