Alright so, for your background, you could use a sprite with animation frames and set the frame to the current level, for skill unlock, you could use a function that trigger when a level is complete and have a condition for each level that unlocks a skill;
Now when it comes to your events logic, try this :
On Left button Clicked + Jugador is SubidaDeLevel : Set SubidaDeLevel to false + timescale to 1 + Set menu invisible