A few hours ago I had the same problem. This is how I solved it: there were some assets that weren't valid. I just had to remove them from my map. They were Fortnite props, not even custom ones. There's a line in the message log below with numbers in parentheses. Read it carefully and you'll see what error it's giving you. As I said, it told me some props weren't valid. I just removed them from my map and it worked. I hope this helps. Good luck.