Update: We have received reports that the Instagram app can once again be installed from the Windows Store
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!
Thanks to our readers for the tips and for the posts in our forums!