Missing storage space in Android 7.0
Never seen this before!
Anyways here is the steps i would do:

1. If you are sure that the 15.9/16GB is wrong, e.g you know you haven't got that many apps/videos, then proceed to step 3.

2. If you know you have got like more than 50 apps or really many videos, take some time and delete videos/apps (of course images, otger files too) that take much storage.

3. Go to Setting > Device Mainteance and then clean everything you can (optimize the phone)

4. If that doesn't work go to like a phone reparator and check what they can do.