Because of the way Facebook permission work, in order for the Ripl app to publish on your behalf, you'll need to connect your personal Facebook account with Admin rights to your Facebook Business Page or Group. You can still select which account you want to share to on the final "caption your post" screen by tapping the "+" button above the keyboard. Any selected accounts will have a turqoise check mark or square next to it. Don't forget to press the final "share" button in the upper right corner to complete your post!
For more information on connecting your Facebook Page in the Ripl app, please see this article: How do I post to my Facebook Business or Organization Pages?