How do I fix animation must be marked as legacy?

I had a problem with this error before. To solve this error, select your animation, and when it shows up in the inspector window, click the icon next to the lock icon and click debug. If it doesn't let you change anything you might want to try clicking the lock button and that might change something. Hope this solved your problem.

you can change animation type from 2 to 1 and check the frame rate i changed it to 30 and it worked.. and from default to looped.

If you don't want to mark your animations as legacy, you can use the new Playables API to play non-legacy AnimationClips:

 private List<PlayableGraph> graphs = new List<PlayableGraph> ();
 
 // Just call this function when you want to play an AnimationClip on a specific GameObject.
 // from https://docs.unity3d.com/Manual/Playables-Examples.html
 private PlayableGraph playAnim(AnimationClip clip, GameObject obj) {
     PlayableGraph playableGraph;
 
     AnimationPlayableUtilities.PlayClip(obj.AddComponent<Animator>(), clip, out playableGraph);
 
     // save all graphs we create and destroy them at the end of our scene.
     // you might need to optimize this if you make a lot of animations.
     graphs.Add (playableGraph);
 
     return playableGraph;
 }
 
 void OnDisable() {
     foreach (var g in graphs) {
         g.Destroy();
     }
     graphs.Clear ();
 }

I fought with this error all day long! Nothing above didn't help me (because my animation was not inside .anim-file but inside .fbx-file) What helped me is this:

1) Delete Animation component

2) Add Animator component

3) Inside Animation tab (don't mix up with Animation component) hit Create button and save Dance.anim file (or use other name).

4) Inside Animator tab select node Dance (name as above) and in Inspector choose your clip from FBX-file for Motion field.

That's all!