Q:

Flipping pose isn't working

when I copy a pose (ctrl + c) and then flip it with this shortcut(shift+ctrl+c) the I see some weird result don't know why.

then this happens after I've pasted it

You can check the blend file if you want. I've uploaded it in my google drive.

A:

Your bone "Arm.R" have wrong rotate.
So,simply to fix it you should delete this bone and all hand bone and add new bone by symmetrize from left side.

