Change your Safari Browser to iphone Size & post to Instagram from your Mac.
Instagram will only allow you to post from a phone by default. You should set your screen size to be that of a smart phone / tablet / device from which you are able to post to instagram.
Open your Safari browser and enable the DEVELOP menu
- Pull down the “Safari” menu.
- Choose “Preferences”
- Click on “Advanced” tab.
- Check the box next to “Show Develop menu in menu bar”
- Close Preferences, the Develop menu should now be enabled and now also be visible between Bookmarks and Window menus.
- Go to the Instagram website and log in to your account in a new Safari Browser tab.
- Then select Develop (from the menu bar, Then click User Agent > Safari – iOS 11.3 iPhone (or correct version).
You can now refresh the page and the instagram page should appear similar to that on an iphone.
You should now be able to publish similar to being on an iphone