Get rid of your action, it’s not good. Your bone rolls are not conducive to pasting a bone “flipped”. If you flip a rotation on local X, then you want it to paste flipped on Z the opposite way. IE, left (-x) to right (+x). So now your bone rolls need to reflect that. Yours do not. You have them going the same way. Your Z axis points to the positive way your bones should rotate on local X. For example your elbow in your blend does not bend like an elbow. Go through and fix that. Line up leg bones correctly, your knees bend an awful way on local X. Make sure your spine bends forward if you rotate on local X the positive way. Same with fingers etc. As you fix one bone, then select the opposite bone and reselect the one you fixed and control N to active bone to get your right and left corrected. That will straighten out the other side as well. You can check this by jumping into pose mode. Rotate the bone on X local (r then X and X). Then copy and paste flipped using the header. If you have done your roll correctly, this will pose your opposite bone like I think you want. If you have trouble post back or pm me.