I'm trying to show a frame on screen for 1 second, why wont it work?

Have you define humanoid and frame? If so then please show them if not then here is what you have to do:

Seat:GetPropertyChangedSignal("Occupant"):Connect(function()
	local humanoid = Seat.Occupant -- This is the humanoid of player sitting on the seat
	local character = humanoid.Parent -- We need character to get player, character is humanoid's parent
	local player = game:GetService("Players"):GetPlayerFromCharacter(character) -- Function that gets the player from character, we need it to access player gui
	local player_gui = player:WaitForChild("PlayerGui") -- All the guis that player sees are inside of this instance, next we need to access the frame
	local frame = player_gui:WaitForChild("FrameName") -- If you want to clone the frame then use :Clone() function
	
	     if humanoid == nil then return end
	    frame.Visible = true -- Sets the frame and all it's descendants not visible
	    wait(1)
	    frame.Visible= false -- Sets the frame and all it's descendants visible
	end)

I hope you understood, i tried to explain from what i've seen in your script but i am not sure what you are trying to achieve and where is the error but this should work.