Error when creating site collection: "There are no inclusions defined for this Web application."

So you're in central admin and click "Create site collection" and you get this error message. This is because you don't have any available managed paths. This is often the case if you remove any wildcard inclusion managed paths and all of your explicit inclusions have site collections in them already.
How do you fix it? You can either create an explicit inclusion for your next site collection, or create a wildcard inclusion managed path.