Why does this script only work in Play Solo?

I've tried everything. Nothing makes it work online. Yes, this is a localscript inside a HopperBin.

plr = game.Players.LocalPlayer
mouse = plr:GetMouse()
chr = plr.Character
o = false
script.Parent.Selected:connect(function()
	plr.PlayerGui.codes.TextLabel.Text = "Press M to see what magics you have unlocked!"
	mouse.KeyDown:connect(function(key)
		if key == "m" then
			if plr.PlayerGui.Magics.Frame.Visible == false then
				plr.PlayerGui.Magics.Frame.Visible = true
				plr.PlayerGui.codes.TextLabel.Text = "Remember to say !save to save tools, and !load to load tools!"
			elseif plr.PlayerGui.Magics.Frame.Visible == true then
				plr.PlayerGui.Magics.Frame.Visible = false
			end
		elseif key == plr.PlayerGui.Magics.Frame.FBK.Text then
			if o == false then
				o = true
				for i = 1,1 do
					game:GetService("Chat"):Chat(chr.Head, "Fireball!")
					local p = Instance.new("Part", workspace)
					game.Debris:AddItem(p, 5)
					p.BrickColor = BrickColor.new("Bright red")
					p.Transparency = 0.3
					p.TopSurface = "Smooth"
					p.BottomSurface = "Smooth"
					p.Size = Vector3.new(10,10,10)
					p.Shape = "Ball"
					p.Anchored = false
					p.CanCollide = false
					local o = Instance.new("Part", workspace)
					game.Debris:AddItem(o, 5)
					o.BrickColor = BrickColor.new("Deep orange")
					o.Transparency = 0.2
					o.TopSurface = "Smooth"
					o.BottomSurface = "Smooth"
					o.Size = Vector3.new(8,8,8)
					o.Shape = "Ball"
					o.Anchored = false
					o.CanCollide = false
					local w = Instance.new("Weld", o)
					w.Part0 = o
					w.Part1 = p
					local f = Instance.new("Fire", p)
					f.Heat = 20
					f.Size = 20
					p.CFrame = chr.Torso.CFrame * CFrame.new(0,0,-5+-i)
					local ow = Instance.new("StringValue", p)
					ow.Name = "Owner"
					ow.Value = plr.Name
					local y = Instance.new("BodyVelocity") 
					y.maxForce = Vector3.new(math.huge, math.huge, math.huge)
					y.velocity = plr.Character.Torso.CFrame.lookVector*80
					y.Parent = p
					local m = Instance.new("Message", plr.PlayerGui)
					m.Text = "Sorry! The fireball skill is still under developement, It will do no damage."
					wait(2)
					m:Destroy()
				end
				wait(0.5)
				o = false
			end
		end
	end)
end)

This is what you did wrong:

When going for the key, you did not key = key:lower(). It is very important, because by default, the keys are uppercase.

So:

mouse.KeyDown:connect(function(key)
	key = key:lower()
	if key == "m" then
		-- code
	end
end

A useful way to debug is to insert print()s throughout the script. For example:

"Why won't my script work?"

local num = 5
if num == 4 then
	-- code
end

"Adding prints..."

print("ok")
local num = 5
print("ok2")
if num == 4 then
	print("ok3")
end

"Oh, that's right. I'm looking for 4, instead of 5!"