How would I add a cooldown to this textbutton script?

Simple! All you need to do is to add a debounce also known as a cooldown.

local b = script.Parent
local f = script.Parent.Parent.Frame
local canClick = true

b.MouseButton1Click:Connect(function()
	if canClick then -- check if value is true
		canClick = false -- set this variable to false so that previous statement cant run until value is true
		f.Visible = true
		
		wait(1) -- cooldown time
		canClick = true -- set value back to true
	end
end)

Hope this helps. If there's anything wrong, do let me know. Remember to accept if it helps. :D