Why can't I rotate this hand upward?

Hello, I made this abstract female figure. I want her to hold the baby. I have an IK on the left arm. Why can’t I rotate the bone Armature mom lower arm.l so that I can rotate the hand to face up with the thumb outward? Thanks. Child one year low poly.blend (1.8 MB)

Parenting on the mesh for the armature points to child not mom.

Thanks for your reply. I’m sorry I don’t follow you. Could you explain?

Select your mesh. Now look at your parent in property object. You have parented the mesh to the wrong armature. You have it parented to the child armature but are trying to move the mesh with the mom armature.

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Are you using R YY to rotate. I’m not having any issue in 2.79.