This forum in particular is mostly to host guides. In the future, feel free to ask in the Discord’s help section. That being said, there’s a limit of 10 active quests at a time, so if you hit that, you’ll see that error message. You’ll simply need to finish or cancel one.
I don’t recall there being a level 50 gift box actually. Breno’s beginner guide covers the level up gifts at the start if you want to check that out as well: Beginner Guide to LifeTO (upgraded) by Breno