Goal Flow is a useful Google Analytics tool allowing you to view the paths visitors take to reach your 'Goal URL' or conversion page. If you're receiving this error 'You may have applied a condition such as a date range, an advanced segment or a goal for which there is no data; or there may not be enough data to show results when sampling is in effects.' when trying to view your 'Goal Flow' it's because you don't have enough data to generate a graph with. Increasing your date range usually solves this problem.
You need at least one goal setup in order to use the Goal Flow graph.
Overview of Goal Flow - lovesdata.com
Here's Google's official link to an Introduction to the Goal Flow Report.