Why isn't my stop (Debounce) working?

The problem is the stop variable.

The condition you set, if not stop then

makes it so that the none of the code that hides the frame runs.

A simple solution is to rearrange how you organized your script. Instead of limiting the elseif condition to run when stop is false, we will allow it to run if the stop variable is true.

Also a tip, you do not have to check for "== true" instead, you use if (condition) then or if (not condition) then.

local frame = script.Parent.Parent.Frame
local stop = false
function click()
   if not stop then
       stop = true
       if not frame.Visible then
            script.Parent.Text = "Close"
            frame:TweenPosition(UDim2.new(0, 300, 0, 0), "Out", "Back", 1)
			wait(1)
			frame.Visible = true
            stop = false
        else
            script.Parent.Text = "Shop"
            frame:TweenPosition(UDim2.new(0, 300, 0, -350), "Out", "Quad", 1)
			wait(1)
            frame.Visible = false
            stop = false
        end
   end
end
script.Parent.MouseButton1Click:connect(click)