I reccomend you talk with the product owner in this situation. Unclear requirements are commonplace sadly so you will need to find out how the user is looking to use this application.
Find out what the happy path scenario is. From there you can start playing around with out to make things fail. Once you understand how the app works in its intended scenario you can then start writing tests that poke and prod unintended scenarios. Look at error handling, performance measurements, and storage capabilitys.