How can I make it so a key opens a GUI? Keeps giving me a nil value

Use UserInputService.

Here's an example.

local Keycodes = Enum.KeyCode --For easier reference.
local Up = true;
game:GetService('UserInputService').InputBegan:Connect(function(Key, Process)
	if (Process) then return end --If the player is in the menu, or a frame with Modal is present, then don't continue.
	Key = Key.KeyCode --Reassign key to it's keycode.

	if (Key == Keycodes.E) then
		if (not Up) then
			script.Parent:WaitForChild('Frame'):TweenPosition(UDim2.new(0,0,-1,0), Enum.EasingDirection.InOut, Enum.EasingStyle.Sine, 1, true) --Tween a frame upwards
			Up = true
		else
			script.Parent:WaitForChild('Frame'):TweenPosition(UDim2.new(0,0,0,0), Enum.EasingDirection.InOut, Enum.EasingStyle.Sine, 1, true) --Tween a frame back down
			Up = false
		end
	end
end)

If something's giving you a nil value, it's most likely the BoolValue. Try using WaitForChild on the "open" variable.