Can someone answer my question on why the ends are never working!?

local Player = game:GetService('Players').LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Character:WaitForChild('Humanoid')
local Mouse = Player:GetMouse()
local Power = game.Players.LocalPlayer.leaderstats.Power
while true do
if Power.Value == "Super Speed" then
	Mouse.KeyDown:connect(function(key)
		if (key == "c") then
			if Humanoid.Walkspeed == 50 then 
				Humanoid.WalkSpeed = 10
			elseif Humanoid.WalkSpeed == 10 then
					Humanoid.WalkSpeed = 50
					
				
			end
			end
			end
	end)

Okay, I think you're a little confused about how the ends work. When you have an elseif statement along with an if statement, it only requires one end, not one for every if.

Here's how you would arrange the ends:

local Player = game:GetService('Players').LocalPlayer
local Character = Player.Character or Player.CharacterAdded:Wait()
local Humanoid = Character:WaitForChild('Humanoid')
local Mouse = Player:GetMouse()
local Power = game.Players.LocalPlayer.leaderstats.Power
while true do
if Power.Value == "Super Speed" then
	Mouse.KeyDown:Connect(function(key)
		if key == "c" then -- You also don't need the parenthesis here.
			if Humanoid.Walkspeed == 50 then
				Humanoid.WalkSpeed = 10
			elseif Humanoid.WalkSpeed == 10 then
				Humanoid.WalkSpeed = 50
			end -- Ends off the inner-most if statement.
		end -- Ends the "if key == "c""
	end) -- Ends the KeyDown event.
end -- Ends the original if statement.

I think for the most part, you're just overcomplicating it, but that's okay, as long as you learn in the end.