Check the chamber and ensure the chamber is empty of any ammo.
Point the weapon in a safe direction and dry fire it(pull the trigger).
Move the slide toward the rear about 1/8" and then:
while moving the slide forward,
hold the trigger rearward.
The slide should come right off. Maybe a NY trigger is installed? Look in the trigger housing mech. for olive green or orange trigger mech under the cuxiform.
NY # 1....................NY # 2