Do I need all the resources in my hold for an NPC mission?

When taking an NPC mission always be aware that the amount of items they are listing as their requirements need to be in your ships store / cargo hold when trying to complete the mission.

So if an NPC needs 100 units of Water and your cargo hold is only 50 you will not be able to complete this NPC mission at this time.

There is nothing stopping you noting down the NPC's current request and coming back later when you do have enough cargo space. If you took the mission by mistake simply go to the top right drop down 'Active NPC Missions' and abort the misison.