How to apply Coupon code 'COUPON20' and offer 20% discount on a product ?

 How  to add Promotions & offer attractive discounts on Products for Customers in your Salesforce B2B Store ? 

Or we can do this by implementing a scenario given below.

Use Coupon Code 'COUPON20' and offer 20% discount on Products.

1. Go to the App Launcher --> Commerce App --> Promotion Workspace --> New
2. If you do not see Promotion Workspace in dropdown, you can edit it and add.
3. Enter Name, Display Name (visible to customer in cart and order summary page), priority no.(which determines which promotions apply first), enable Active checkbox.
4. From Qualifier criteria dropdown, select any/all of the promotions.
5. Enable Commerce Promotion checkbox.
6. Enter start date, end date.
7. You can select Promotion is automatic checkbox but as I want to apply coupon code manually to the Product, so leaving it unchecked.
8. If you do not see any of the field like commerce checkbox, automatic checkbox etc., then go to the Promotion object, check FLS, enable it and save it.
9. Assign this Promotion to Segment, go to the segment tab--> Add --> Enter Name --> Save.
10. Go to Segment's Related tab --> Assign Store and Buyer Group --> Save.
11. Go to the Discount tab --> New --> Select Target field --> You can select individual products, categories or empty --> Select Target Type that matches your target --> Choose Adjustment type by selecting percentage discount or a fixed amount(Enter 20% discount).
12. Qualifier's tab --> New --> Add it Qualifier field.
13. If you Qualifier field is empty, you can select Transactional Total --> Enter Minimum Quantity/Amount --> Save.
14. Create a Coupon, go to Coupon's tab --> New --> Enter Code ''COUPON20''(used by the customers to apply it as a coupon code) --> Set it Active --> Enter Start date and End date --> save.
15. Now we need to customize Promotions in experience builder of B2B Store.
16. Enter 'Cart Page' at the top-left and drag coupon component from the left side(from Components) and add it below Coupon component.
17. Repeat the above point with Order Summary page.
18. Publish your Store.
19. Now, log in to Store via Contact --> Add Product to the Cart (keeping in mind the minimum quantity or minimum amount) --> Now you must be eligible for coupon --> add coupon code and apply --> You can see discount amount getting deducted from the total amount --> Checkout --> View Order Summary page

Reference : 

https://help.salesforce.com/s/articleView?id=sf.comm_promotions_get_to_know.htm&type=5