Why can't you push the plunger into a syringe if your finger is over the opening at the front of the syringe?

When your finger is over the opening of the syringe, it creates a sealed environment that prevents air or fluid from escaping. Consequently, pushing the plunger compresses the air or liquid inside, increasing pressure but not allowing any outlet for the force. This pressure buildup makes it impossible to push the plunger down effectively. In essence, the closed system prevents movement due to the lack of a release path for the pressure.