GSX Airport Services don't have any relationship with the ability to have services in GSX. In fact, they don't have anything to do with GSX, since they control only the look (operators, basically) of vehicles that are used as default Ground Services, that is when *not* using GSX.
When GSX proper returns that error, the problem is invariably some issue with the scenery, like orphaned parking spots, missing links, wrong links, etc. Nothing you can fix from GSX, you must report the problem to the scenery developer.