How can I tell if an animation can be used by animator? And how to create this kind of animation

I was trying to add some stuff to the default animator. So I bought some humanoid animations from Mixamo Store( a plug-in for unity, you can preview and buy animations there). When I previewed those animations, they worked very well for my model and I'm pretty sure they can be applied to humanoid model with bones. However, when I downloaded them and try to applied them to my animator, the system says these animations need to be set as legacy... So how can I tell if a animation can be used in animator? And more important, I'm using 3d Max so I hope to know how can I create and export animation that doesn't need to be set as legacy to use?