It seems like the quantity of the products is in negative or insufficient stock, so that product is not allowed for billing.
We kindly request you to stock in the product by following the steps provided in the article below:
Mobile: Stock in inventory for individual products in mobile app
Web: How to do stock in for the products on Web?
Or
We suggest you to enable the allow negative quantity option to create a bill when quantity is zero. Please follow the steps mentioned in article below:
Mobile: https://community.getswipe.in/t/how-to-allow-negative-quantity-in-mobile/5