The problem is you are applying :MoveTo() with the wrong parameters as. Your script only runs once as well and doesn't start the while loop.

Humanoid:MoveTo(location, part)

The first parameter, location, is a Vector3 value. This is where you want to humanoid to go. The second parameter, part, is sort of like an offset to the location. If you click the wiki page, you can see a gif that explains it better.

If I were to pass a part with the location being Vector3.new(0,0,0), the Humanoid will walk to the part's location and will update when the part's position changes.

Note: The Humanoid will timeout after 8 seconds of being unable to reach to destination. To avoid this, keep resetting the timeout by running the function again. More info on the wiki page mentioned above.

Making the NPC Better

Now that we understand the :MoveTo() function, what are some ways we can apply this to our NPC?

PathfindingService As much as I would like to explain this, I never have used this before. What it does is it creates a path for the NPC to follow based on the geometry of the map. Here is a tutorial on the wiki page: https://developer.roblox.com/articles/Pathfinding

Chasing the Nearest Player Using Magnitude to get the distance between two points in studs will help us determine what player is closest to our NPC and we can apply :MoveTo() to have the NPC chase them.

local npcHRP = NPC.HumanoidRootPart

local function GetNearestPlayer(minimumDistance)
    local closestMagnitude = minimumDistance or math.huge
    --minimumDistance is a number in studs
    local closestPlayer
    for i,v in next, game.Players:GetPlayers() do
		local Character = v.Character
		if (Character) then
			local humanoid = Character.Humanoid
			local HRP = Character.HumanoidRootPart
			if (humanoid.Health > 0) then
				local mag = (npcHRP.Position - HRP.Position).Magnitude
				if (mag <= closestMagnitude) then
					closestPlayer = v
					closestMagnitude = mag
				end
			end
		end
	end
	return closestPlayer
end

Using this function, we get the nearest PlayerObject. We can 'convert' that to a Vector3 and run the :MoveTo() function with it.

Note: Make sure you apply the loop to keep checking for the nearest player. You also need to keep applying :MoveTo() to keep the NPC from timing out.