Hello, @TFS,
1. If you are using the “Product Layout Builder”, then you have to go to XStore >> Theme Settings >> WooCommerce >> Single Product Builder >> Add to Cart & Quantity and turn ON the “Sticky Add To Cart Bar” from there: https://postimg.cc/BtbyQ9B7
2. If you are using the “default/prebuilt” layout for the single product page, then you have to go to XStore >> Theme Settings >> WooCommerce >> Single Product Page >> Single Product Layout and turn ON the “Sticky Add To Cart Bar” from there.
Regards 8Themes Team.