Can I show recommendations on checkout pages?
Unfortunately, Shopify apps cannot run on checkout pages and for that reason, the app widget won't show up on the checkout pages.
This is a Shopify intentional policy and no app can change that without breaking the Shopify 'terms and conditions' at https://help.shopify.com/en/api/app-store/listing-in-the-app-store/app-requirements#a-security
Please note that the situation is the same for all Shopify apps, not only Frequently Bought Together.
The app widget can be shown in product pages but not during the checkout process.