First, double check that your promo code date has not expired. If the date is still valid, you might also need to ensure that the promo code covers the full value of your course. For instance, some codes are worth $30 USD and others are valued at $90 USD. Check the details on your offer for more information.
If you’re still having problems, check your spelling, make sure that you’re not entering the code in the online shop payment cart, and ensure that you’re not attempting to apply a DLI Self-Paced Training promo code for a DLI Instructor-Led course.