So, the tutorial and documentation seem to omit the fact that you need a battle pass or enough Fortnite experience or something to build an island of your own. This is an understandable oversight, because the people you’re primarily interested in having as creators are already in the game. But as someone who wasn’t a Fortnite player before… It was confusing to launch a test session UEFN and have it fail to create a session.
Hey there and welcome to UEFN! I work on the UEFN docs team. This is great feedback, we’ll update our documentation to avoid confusion amongst newcomers in the future
Hey there 
I looked into the issue you experienced and since the time you onboarded into UEFN the restrictions have been eased to reduce frustration and limitations. You can find more information here: https://create.fortnite.com/news/fortnite-island-creator-program-eligibility-change
If you still experience problems, please reply here with details about what type of error messages you received, and any blockers you experienced.
Thank you again for your feedback!
Are you blocked in trying to publish your island or play test your island? You should be able to playtest your island. Can you try again and give us any errors you encounter? If you can report the errors that will help: https://create.fortnite.com/news/using-the-creative-and-uefn-bug-reporting-form?team=personal