Hi, I'm not sure I have understood your problem. You can build the stable as soon as you have enough money and then continue the quest. If it is a time limited quest then you may lose the extra prize, but this will not prevent you from continuing the main quest (are you doing the Need for steed quest?). Hope it is clear 🙂
I am afraid there is no way around it. You should have completed the Need for steed quest before starting the Superpower ultra quest (and you need a toddler too). In order to avoid this kind of problem I strongly recommend you to have a look at TheGirlWhoGames website. You can find all quests explained step by step. It is really really helpful!!! 😉
You need to contact support at this link. http://answers.ea.com/t5/The-Sims-FreePlay/INFORMATION-The-Sims-FreePlay-Support/td-p/1877383
