You are trying to add (or at least, iTunes is trying to add) more than can be fitted onto the iPod. The message will also tell you how much additional space is required in order to fit everything you have selected. In this event, iTunes will fail to perform the Sync.
If you say that you have 19 GB available, then you're trying to add more than 19 GB of media. Don't forget, that media may include music videos etc.
You will have to establish exactly what you're trying to add so that you can determine how much space is required, then take action to remedy whatever is causing the issue.