"Mesh contains Root bone as root but animation doesn't contain the root track"

[anonymous_user_49a519741 (anonymous_user_49a51974)]

You need to go through the painful process of making the external skeleton compatible with yours. The process involves matching the bone names in the external skeleton with the bone names in the skeleton you want to retarget the animation to. This got me through the process, eventually: https://docs.unrealengine.com/latest/INT/Engine/Animation/RetargetingDifferentSkeletons/index.html