See the video. Everything appears to be setup correctly. I added the weapon_r bone to the target mesh. In the retargeting preview, the bone is correctly being retargeted. But when I export the animation, the bone is untouched. Is this a bug?
It looks like the target has the weapon_r bone, but does the source have the same bone?
Does your Op Stack use FK or only IK?
It also looks like you got a log of errors most of which I can’t see.
They both have all the same bones. As you can see in the preview, everything appears to be mapped correctly. I did a test where I added the bone in Blender, and it worked. So this appears to be an issue when adding the bone via Unreal, it fails to retarget that bone.