Hi there, @Kristin_G and welcome aboard 
The thing is that there is a restriction coming from Instagram. They don’t allow posting from third-party apps, that’s why this button isn’t available in our app.
However, we keep tabs on this situation and have this idea on the Wishlist. You can upvote it here - Requesting Instagram added to Social Sharing
If any news comes up, we’ll update you on the Wishlist thread 