How do I lock level buttons until a checkpoint on the last level is complete?

I am trying to make a save game, so that the level select locks the buttons until the checkpoint on the previous level is made. Could you help me out with this please? Thanks!

You can store persistent data in the GameInstance blueprint. This is the only one (if I’m right) to keep data until the game is closed.

for saving system (when the game is closed / reopened) look at this : https://docs.unrealengine.com/latest/INT/Gameplay/SaveGame/Blueprints/