I had this same problem, and for me the problem turned out that I had the wrong skeletal mesh selected, as in not the same one as in my Animation. When I imported the skeletal mesh, it also created a named_skeletonGroup36960 and a named_skeletonGroup36960_PhysicsAsset.
I had chosen the Group one instead of my actual asset.
This kills the skeletal linkage so it can’t use the animation.