I believe "IP Set Exceeded" means you do not have sufficient licensing to add additional sets. You could go into Maintenance and see the status of the sets and delete some that are offline but be vary careful and do your research first, sometimes IP sets are used for routing or mailbox purposes and do not have sets actively on them but still consume licenses.
If you need additional licensing, you will need to contact the vendor that holds the license record for your PBX, and if you don't know who it is you can have any vendor request to take control of your license record... Be aware the original vendor will be notified this is happening and may have a right to protest the change over if you are in a contract or not "in good standing" with them (meaning, you don't owe them money).