My doubt is can I use sustainabilty as a need to be addressed or is it too general?
Sustainable development is a large topic you could make an app but it will take time and considering that the 4th day is going on I would recommend choosing one of the goals also see this -
This is Theme # 3 and the topic certainly includes sustainability. Too general? You can present the topic as a renewable water supply; crop management, sustainability is avoidance of the depletion of natural resources in order to maintain an ecological balance. Pick an area of conserving natural resources and you should be fine. Make an app about what you would like to do keeping in mind you have a time limit.
Theme 3: Computational Action App
How could your app make the world better? Identify a need in your community and create an app to address that need. For instance, Is there an app that could address water/air quality? Is there something that could help with the problem of food insecurity? Can your app help with demands in a post-Covid world? In your write-up and video, give us a clear, concise description of how your app addresses a community need.
I agree with @SteveJG that you could cast sustainability as a topic for community action. It might be helpful in your description of your app (either the written description or video) why your community (as you define it) would benefit from the app w.r.t. sustainability.
thanks for your advice sir!