I'll try to explain what the Snapchat story red exclamation mark is.
There is a little bit of dispute online about what the red exclamation mark means on Snapchat, but it seems that it means that the person whose story you are trying to view is not suitable for under 18s.
Some people online have also been saying that it means a snap hasn't sent. But I think that only applies to the snaps that you sent, rather than ones on other user's stories.
I've tried to find some pictures to help illustrate what I'm saying, but it's really difficult to find any!
So to sum up: the Snapchat story red exclamation mark is supposedly to warn the user that the story contains content that is unsuitable for children.