Riggers rig a Model to the Epic skeleton, but the APOSE the Modeler creates is not always epic meaning not close enough to the mannequin
On one I found I had to raise the upper arm 10 degrees and the forearm 5 degrees. So you have to match the APOSE exactly(98% lol) in the Retarget Manager by selecting joints and rotating them.
I find it easy to create a blueprint add 2 skeleton meshes then select 1st SK and set it to the mannequin then second one to your SK
You can see in 3D the difference in the arms.
Sometime shoulder height is a pain but thanks to the blueprint you can scale one of the chars to align shoulder but I just wing it knowing only the angle matters.
You can create a Animation Asset in the same place Retarget Manager is, from the pose once you start adjusting and then in the blueprint set the animation asset and you can see your changes, You will notice you may adjust and the hand is now to far the other way this means you adjusted too much.