Today we published a new tutorial on how to solve the FBX Import Error when importing a new Mixamo animation:
FBXImport: Error: Mesh contains root bone as root but animation doesn't contain the root track.
Import failed.
For more details, you can also read the detailed steps in the official documentation: https://www.unamedia.com/ue5-mixamo/docs/fix-fbx-import-error/