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 do not have available addon slots. In order to set a new an addon domain you should first purchase an Addon Slot from your Customer's Area -> the Order tab -> Order Additional Features For Existing Account -> Addon Slot.

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