Before you can start posting your Shopify products to Facebook with the Sonlet Shopify app you'll need to add the Sonlet Facebook app to your Facebook groups where you'd like to post. This is best done from a computer, on Facebook through your browser.
In summary, you can add the Sonlet Facebook app to your group by going through the group's "Edit settings" dialog. A detailed walk-through with screenshots is below.
First, go to your group's edit page by clicking "Settings" on the group page. Then scroll down and click the edit icon next to "Apps".
Next, use the search field to find the Sonlet app. Once it appears in the filtered search results, go ahead and click it, then click "Add".
Now Sonlet should display in your apps:
(You can view Facebook's instructions for installing an app in your group here.)
This process should make it possible to post to Facebook. Please don't hesitate to reach out if you have any questions along the way.