I been reading up and this is a bug that others have had. First try killing the boss again. But if that doesn’t work the only way that someone was able to fix it was to have a GM reset the quest to kill Xan and make Gara tameable.
Just FD to reset encounter same with pets if you have them out. Also its been so long but you might be taking to long to kill. In the encounter the void lord is consuming Gara’s soul idk if thats interuptable but it is an epic chalange mode tame they are not ment to be easy. Also cant remember if it scales with lvl and you would have to be in BM spec to tame a spirit beast as well which could be the problem. Idk if you recieved a wow txt that you must be BM to tame or lack the proficiency to tame this pet. There was also some mention of not getting a debuff in the guide that might be adversly affecting your tame.