To enable discounts, go to Settings > General > Allow product-level discount.
Here's how to give a discount on the product level:
Add the product to the cart
Tap the product and you’ll now see 3 discount options:
Add a new price for the product
Add a discount percentage for the product
Add the discount amount
Once you’ve added your discount you can choose the right product quantity, add a note and save the discount.
You can now proceed with the payment.