If you're here, it's probably because you can't complete the WoW All Terrain Snail Quest. Well, it's not your fault. As of now, it seems that it is still impossible to perform it and a user on WoWhead shares the solutions he tried.
Disabling all addons
Abandoning & Picking up the quest again
Ride Slick out of the zone and ride them back in
Perfectly aligning Slick on one of the markers
Doing a lap first (as the quest states)
Deleting Slick from my bags & Getting a new one
Reloading while on Slick
Few days ago, Blizzard notified players to be aware of the issue. It should therefore be fixed as soon as possible.