I get a warning that I can't level up before I have bought and completed existing challenges (The Beginning). My choices are "keep mining" and "buy challenges".
However, I do not have enough cristals to buy any of my available challenges. I would expect the game to no this and not offer to go and buy them when I can't afford them. Maybe an alternate message like "level up frozen until existing challenges completed" or something? (I personally wouldn't need to see it each time, either.)