Game over

The final task for your UI is to react to the game ending:

func show_game_over():
    show_message("Game Over")
    yield($MessageTimer, "timeout")
    $StartButton.show()
    $MessageLabel.text = "Coin Dash!"
    $MessageLabel.show()

In this function, you need the Game Over message to be displayed for two seconds and then disappear, which is what show_message() does. However, you also want to show the start button once the message has disappeared. The yield() function pauses execution of the function until the given node (MessageTimer) emits a given signal (timeout). Once the signal is received, the function continues, returning you to the initial state so that you can play again.