Brakes - 2003 mustang rear discs

I have removed the calipar housing on my rear brakes to replace the pads I cannot get the piston to go in so as to put the caliper housing back on the disk. I have bled the system and tried with a c clamp to get the piston i, it will not move.

The caliper piston has to be rotated back into it’s bore. Do you see any slots or notches on piston face? The tool above engages those slots/notches to rotate the piston back in.