For information of retargeting animation, see Animation retargeting and Retarget animation from one skeleton to another.
Error: Skipping arm retargeting since arms are not labelled
Some of the joints in your arm joint chains are not labelled. All joints, including all those in the hands, must be labeled for a successful retarget.
See Label a skeleton’s joints and Skeleton > Joint Labelling > Add Retargeting Labels.
Error: Error finding matching corresponding joint in n chain
Where n represents a limb type such as Left Leg or Center Head.
One of the joint hierarchies (source or target) that you selected for the retarget has a different number of limbs than the other. The retarget solver can not solve between skeletons with different numbers of limbs (for example, single-headed source skeleton > two-headed target skeleton).