First, check if the product is available in your store or not if not then upload the product in your store and then upload products to Facebook.

Step 1: Upload the products on Facebook then select the products > more actions and click on Make products available.

Step 2: Select the name of the product that you want to show on your Facebook Shop section.

Step 3: Click on Manage, from the sales channel section of the product details page.

Step 4: In the Manage sales channels availability dialog, check Facebook, and then click Done.