Health drain power wont work why?

Well, you have some problems and missing components.

First, this is all local so you will only be damaging a player on your screen, but in reality, nothing is happening to them on the server's side.

I also don't see how you're getting a hold of the targeted players humanoid so I also believe you're missing that. I'll walk you through how to make it on the server's side, which will require a RemoteEvent and a ServerScript to be added.

Please keep in mind that I took the time to make a new LocalScript, along with a server script to guide you to what your goal is, so anything unfamiliar to you will be expected.

1. Let's add a RemoteEvent in ReplicatedStorage and name it HealthDrainEvent

Now, the LocalScript in StarterCharacterScripts will contain: The LocalScript has been edited to use a key for it to activate instead, using UserInputService to retrieve the input from the player

local RP = game:GetService("ReplicatedStorage")
local UIS = game:GetService("UserInputService")

local HealthDrainDevent = RP:WaitForChild("HealthDrainEvent")

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()

local draining = false
local cooldown = 1


UIS.InputBegan:Connect(function(input, processed)
	if input.UserInputType == Enum.UserInputType.Keyboard then
		if input.KeyCode == Enum.KeyCode.E then --Change to whatever key you want
			local target = mouse.Target

			if target.Parent.Humanoid or target.Parent.Parent.Humanoid then
				local TargetHumanoid = target.Parent.Humanoid or target.Parent.Parent.Humanoid

				if TargetHumanoid ~= player.Character.Humanoid then
					print("Enemy humanoid detected, initiating doom.")
					draining = true
					while draining == true do 
						HealthDrainDevent:FireServer(TargetHumanoid)
						wait(cooldown)
					end
				end
			end
		end	
	end
end)

UIS.InputEnded:Connect(function(input, processed)
	if input.UserInputType == Enum.UserInputType.Keyboard then
		if input.KeyCode == Enum.KeyCode.E then --must correspond to the key you use to active the process
			draining = false
		end
	end
end)

and finally, add a ServerScript in ServerScriptService and put in the following:

local RP = game:GetService("ReplicatedStorage")

local HealthDrainEvent = RP.HealthDrainEvent

HealthDrainEvent.OnServerEvent:Connect(function(player, targetedHumanoid)
	if targetedHumanoid.Health > 50 then
		targetedHumanoid.PlatformStand = true
		targetedHumanoid.JumpPower = 0
		targetedHumanoid:TakeDamage(2) -- or whatever damage you want
	else
		targetedHumanoid.PlatformStand = false
		targetedHumanoid.JumpPower = 50
	end
end)

What you've been trying to achieve should now be working, make sure to alter it to your needs, for example, if you want them to press a key in order for it to activate instead of clicking. At the end of the day I hope this helps you out and if you have questions please add a comment below this answer.