How do I reference an object using a string?

I am building a level navigation menu, so here's how it works.

When the player completes a level, I add 1 to a variable called LevelsCompleted.

If a level access button's level number is equal to the number of levels completed + 1 (because there is already level 1), I set it visible (if it's not, I set it invisible). But the problem is, I have to reference the level button using a string ("Level" & LevelButton.LevelNumber), how do I do that.

I don't understand your setup. Are level buttons instances of one sprite? Or are they different sprites?

The easiest way is to make one sprite object LevelButton, and add "Level" instance variable to it. Manually set level numbers to all buttons in the layout editor. Then you can use "LevelButton compare variable" to reference them.

For example:

LevelButton compare variable Level>(CurrentLevel+1)
	LevelButton set invisible