You are applying a discount that doesn't apply to any of the transaction items.
Discount is not eligible for the provided items
Discount has restricted_to set to certain price_id(s) and/or product_id(s).
Verify you've added the items that match the price_id(s) and/or product_id(s) to the transaction.
Apply a different discount that doesn't have these limitations.
Add different items to your transaction.