An app may not be available for several resaons.
- Minimum requirements - Some apps which require API's released in Redstone or Aniversity update will showup in store and cannot be installed on Threshold(General build at the time of question).
- Missing additional hardware - For example, the app in your screenshot requires bluetooth 4.0.
- Region restriction - Some apps will be released to a specific country or region. As far as I see, many such apps are restricted to US. Try changing your region to US or any other European country.
- Is it released - Many apps appear on the store before they are released. The VLC appeared in the store during private beta but only the beta testers were able to download it. Your screenshot shows that there is no rating for the app. Is the app released for public?
- Is there a another UWP app - Some developers have two apps in the store. One UWP app and another for 8.1 users. In such case a 8.1 may be locked by developers for devices running Windows 10.
- Not for your device - With the unified store you can browse certain apps that are for other devices such as apps for PC sometimes. It does not happen always.
- Pulled from store - Some apps are being discontinued by the developers. Such apps will still appear in store but cannot be installed.
If any of the above reasons are valid you cannot install that app on your current device. Joining the Insider program and private beta program may unlock some apps(which are locked for reasons 1 and 3).