Just follow this advice:

> Please clarify your specific problem or add additional details to
> highlight exactly what you need. As it's currently written, it’s hard
> to tell exactly what you're asking. See the [How to Ask](https://askubuntu.com/help/how-to-ask) page for help
> clarifying this question. If this question can be reworded to fit the
> rules in the [help center](https://askubuntu.com/help/closed-questions), please [edit the question](https://askubuntu.com/posts/986295/edit).

This message tells you what you can and need to do in order to get your question reopened:

 1. You should get the [Tour](https://askubuntu.com/tour) to make yourself familiar with the site's format.
 1. You should read [How do I ask a good question?](https://askubuntu.com/help/how-to-ask) to make sure you make your question a valuable one.
  1. [Search](https://askubuntu.com/search?q="AppStream+cache+update+failed"), and research  
In your case it would be helpful to search for the main error message – `AppStream cache update failed` – on [AU](https://askubuntu.com/search?q="AppStream+cache+update+failed") as well as [the internet](https://www.startpage.com/rvd/search?query="AppStream%20cache%20update%20failed"&language=en) in general. Try to get to the bottom of the problem and include the *whole* result of your research in your question. Please do *not* use comments to add information, instead [edit the question](https://askubuntu.com/posts/986295/edit) directly.
  1. Be on-topic ✓
  1. Be specific  
If you find other resources which cover your problem (like https://askubuntu.com/questions/761592/unable-to-apt-get-dist-upgrade-on-a-persistent-ubuntu-16-04-usb in your case) and they don't solve your problem, include them and explain how *exactly* the solutions proposed there don't work for you. Show us what you tried.
  1. Make it relevant to others ✓
  1. Keep an open mind  
Believe it or not, but we're really trying hard to help you here – we just need you to help us help you.