Help with a quest(horse quest)

Hello, 

I have a question concerning the "horse"quest. I have accidentally repair the stable before the task was assigned to me. And now that the task is assigned to me and I need to complete it to finish the quest, I am not able to press "repair stables" anymore. What should I do? How can I finish this quest in order to move forward to other tasks?I really need help please.

Thank you.