How to make it when a key is down one function goes on and when the same key is up other one does?

This is a local script that is supposed to trigger one sever event if the key q is down and the other one if the key q is up, but all it does is activates the first event. Here is the script:

wait()
local Players = game:GetService("Players")
local Player = Players.LocalPlayer
local Character = Player.Character
local Mouse = Player:GetMouse()
local event = workspace.Events.DemonAbilities["Dark Nebula"]
Enabled = true
ButtonDown = true

Mouse.KeyDown:Connect(function(key)
	if Enabled == false then return end
	
	
		Mouse.KeyUp:Connect(function(key)
		
		key = key:lower()
		if key == "q" then
			ButtonDown = false
		end
		
	end)
	
	key = key:lower()
	if key == "q" then
		
		Enabled = false
		
		if ButtonDown == true then
		
		event:FireServer()
		
		
	
	
	end
	end
	
	

		if ButtonDown == false then
		
		
		workspace.Events.DemonAbilities.darknebulaweldoff:FireServer()
		
	end
	

	
	
	wait(15)
	Enabled = true
	
	
	
end)

Using the KeyDown event from the Mouse is deprecated. See this to know how deprecation effects your code.

You should instead use UserInputService to handle player input.

Example code of what it would look like in your case:

--Get the service.
local UIS = game:GetService("UserInputService")

--Make a function and use the first parameter for the input object.
function onPlayerInput(key, processed)
	if key.KeyCode == Enum.KeyCode.Q then
		print("Letter Q was pressed!")
	end
end

--Connect our function to input began.
UIS.InputBegan:Connect(onPlayerInput)

Note that there is also ContextActionService that allows you to bind more keys and gamepads to functions.

Edit:

For checking if the Q key was released you would use the InputEnded event.