Just a few ideas of where to start.
How is the filter in the tank?
If you take the suction hose off at the pump will oil flow out of the hose?
If you take the pressure hose off at the pump, will oil flow out on it’s own? How about with the engine running and the PTO engaged?
Are you sure the tang on the pump is connecting with the slot in the tractor?
Have you checked the shaft in the tractor to make sure it is turning with the PTO engaged and the engine running?
Also, with the PTO engaged and the engine turned off, you should not be able to turn the end of the shaft that engages the tang on the pump. If you can, something in the tractor is broke.