Many push-fit fittings are not designed to be removable. If that were a sharkbite connector, you get get a special removal tool.
How do I disconnect SharkBite push-fit fittings?
The only way to disconnect the SharkBite push-fit fitting is with a
SharkBite Disconnection Clip or SharkBite Disconnection Tongs. With
the clip or tongs apply pressure to the release collar; this releases
the grab ring teeth. At the same time the pressure is being applied,
pull on the tube with a twisting action to remove the tube. Ref:
With other fittings, the one way metal barbs inside may not have a release mechanism and, if that's the case, your only option is to cut the pipe (or elbow).