Level 6 15 and 37 (and more of same quest) are impossible to do. The quest is that u must sell (amount) of items in order to complete it. But instead of selling a inventory worth of items in the mines it only shows that u sold it once. Ex: instead of 2000 sold items (when u click slime) it only shows 1 item sold. People have been able to complete level 6 and level 15 by using a glitch ( buying things from /shop and then throwing the items away or using /clearinventory ) but that would soo long to do on level 37 (500000 items).