“No” removes all constraints. And is necessary when switching the types of animations you want the final result to follow.
The base process is to get the rig to follow Ue4 anims. Key all significant frames. Click NO to remove constraints, and click “my” to get the rig to folllow the manually keyd animation from before.
Ue4 button is disabled if the FBX file you import doesn’t contain an animation.
Specifically re the error, it seems you moved things away from the collections they need to belong to.
if nothing is within those the system can’t possibly function.
If you play/mess with constraints you need to be 100% sure what you are doing.
connecting the left hand to the right hand is fine, but you should then manually key Loc Rot Scale for that bone and remove the constraint prior to moving forward with baking the animation down.