Problems with Paste X-flipped pose I'm trying to copy the first frame to the last frame by ctrl c and shift ctrl v but it doesn't work well.. how can I fix it?
Your LowerLeg.R IK Pole Angle is bad, it should be 0:

Also, move the knees a bit forwards so that Blender knows in what direction they are supposed to bend:

When you'll have done that you'll need to redo the poses.