That means that either the path (file address) for a recording, or the ingest server address (URL address) for a streaming service has not been properly specified.
If you've chosen a streaming service from the list, then something is preventing your computer from reaching it, or you've not entered a stream key. Or, if you've added a Twitch account directly to OBS, it's possible you don't have 2 factor authentication on in Twitch (you need to have this on).
Are you recording? Streaming? If so, to where?