So I made the mistake of setting a new rest pose so my characters hands weren’t mirroring each other, and now I can paste the pose mirrored on the x axis. All the solutions I could find said just start the rig over, but I have spent like over twenty ours on this thing and I refuse to do it again, especially with the weight paint. So does any one have a solution to make it so I can x-flip poses?
not 100% sure about your case.
what i would try is
goto pose-mode, press a to select all,
pose menu → clear Transform → all
that should return the rig to default T-pose where the mirror would work.
Thank you so much. Though I already Just delete the arms on one side and symmetrized them. Still that advice will come handy in the future for quite a few problems I regularly run into.