Stay Back?

Im trying to get to the level 41-50 grinding spot near cords 100, -400 and every time I try to enter it drags me back saying "stay back" in red letters.

Do I have to activate a quest to enter this area?

 

This is most likely the area where you begin and do the quest "A Grave Mistake", which is level 50 (and it requires you to complete the level 49 quest "House of Twain" in order to start it despite not directly telling you). However, assuming this is indeed appearing when you try to access the back of the mansion, I don't know why you would be trying to access a grind area here, as the only mobs that spawn are mini boss like mobs that spawn via tripwires.