Hi Ameer
You are right pressure is force per unit area but when talking about a system the pressure remains constant everywhere throughout the system eg. if you consider a pressure vessel connected with nozzle and a pipeline you will see the pressure will be constant thruought the system when its sealed by a valve. Now if you consider flow process pressure drop occurs due to resistance to flow, if you reduce the area of flow it increases the pressure drop.
you have mixed these two different effects.
thus if you consider a closed system there would be no pressure change or drop wether its a nozzle or a tank.
drop is only due to resistance to flow.
Hope this clears your doubt