I created a cylinder button in the workspace and added proximity prompt and it triggers a tweenposition frame but when I pressed the button, I don't see the frame on my screen...
Frame Properties: Visible: Yes, Original position: 0.5, 0, -2, 0, Zindex: 3
local spawngui = game.StarterGui.MainMenu.SpawnSelect
prompt.Triggered:Connect(function()
spawngui:TweenPosition(UDim2.new(0.5, 0, 0.5, 0), "InOut", "Elastic", 1)
end)

3 comments
game.StarterGui.MainMenu.SpawnSelect is not equal to game.Players.CedCed6.PlayerGui.MainMenu.SpawnSelect
there is nothing wrong with your script, however, it changes the initial state of the gui that gets copied to eveyrone that joins, therefore, not the gui of the people that have the gui
^ to add you should see the changes if you reset your character after triggering the prompt which will affect all players thats given that UI, the Triggered event you are using conveniently parses the player that pressed it if you see https://developer.roblox.com/en-us/api-reference/event/ProximityPrompt/Triggered