You can't. There are no dead ends in the game, but there is also no restarting on the same account. Don't worry: even if you make some mistakes in the beginning, you can always develop your Castle up to a higher level and fulfill all of Oberon's Tasks. Be sure to follow the instructions in the order that they're given or you'll slow down your Castle's development by using up your resources too fast. If this happens, just wait until you collect enough to continue.