An increasing amount of Windows 10 Mobile users are stating they can no longer install the official Instagram app, due to what the Windows Store calls "hardware restrictions".
Windows Central has received a number of emails from users about this Instagram install issue, and it's also being discussed on our forums. At the moment, there doesn't appear to be a specific reason for this issue to be occurring, but we will update this post if and when it is resolved. If you are also experiencing this issue, please let us know in the comments!