It’s not a mistake, it’s a by product of how UE4 handles bones vs. how Blender handles bones (for instance the huge bone in between his legs is the ‘root’ bone in UE4). Blender creates crazy rotations and scales when you import an fbx file. You’ll basically have a hard time animating THAT skeleton unless you use Maya (I think Max and a few other programs will work as well). There’s nothing wrong with it though. If you export it with the right settings it will look right in UE4. There are a ton of posts concerning this, but you can in theory add IK and such then TRY to animate it but I have yet to see where anyone has gotten it to really work well using Blender. You can, however, create your own skeleton and export it and it will work fine UNLESS you need it to work with that particular UE4 skeleton in some way (as in retargeting). It’s a weird thing, you can work with that skeleton by skinning your character to it - OR - you can create your own skeleton with it’s own animations and use that - but you can’t mix and match - and both solutions require a lot of work.