Basically what is said in the title. I encountered this before, but when there were actually all buildings at or above 400 in numbers. Now I am far from that. I used Spontaneous Edifice about an hour ago without any problem. Yes, I have a saved file, about a week ago. But as that was just before clicking Legacy, I hope I don't have to go through the build up again.
Edit: My mistake, I didn't have enough cookies for activating the Edifice. Now I feel stupid. But I will leave it here, just in case someone else makes the same mistake :D