The app's discount is active. In your Shopify admin, go to Discounts. The automatic discount XBoost Product Upsell must be Active. Do not delete it.
The offer has a discount. In the Offer step, check Automatic Discount (Product Page Upsell) or Enable discount and Discount type (Product Add-Ons).
The shopper added the product from the offer. The discount applies to products added with the offer's Add button or add-on checkboxes, up to the Max Quantity.
Other discounts. Shopify decides which automatic discounts and discount codes can be used together. Check the combination settings in Discounts.