Instagram works a bit differently than other social networks, and not all posts can be published automatically. Buffer offers both direct scheduling (posts that can be published automatically) and reminders (posts that you’ll receive a push notification for). Learn more about this here.
If you would prefer for all posts to be scheduled and sent as reminders, you can control this behaviour in the settings area.
Select your Instagram account on the left hand side of your dashboard. Click on Settings and make sure the General tab is selected. Under "Set Instagram Reminders for all posts", toggle the Disabled option to Reminders only.