If you're unable to use a customer's discount code, you can check its settings from Studio Setup > Discount Codes.
Check to make sure the item you're attempting to use it on is listed in the items it applies to. Assign items at the bottom of this page if needed.
Check that its Usage Limits or expiration dates are accurate. If any changes need to be made, click Actions > Edit.
Additional Settings to be aware of
Global usage: how many times can this code be used among your entire customer base (ie. if you put a 10 here, and a 1 for 'usage per customer', then after 10 separate customers use the code, no other customers will be able to use it).
Auto renewing subscriptions limit: if the code is used on an auto renewing subscription, does the discount apply to all future renewals of a customers subscription, or just a limited number? Put 0 if it shouldn't apply to any renewals.
Test it out
After saving changes, start a POS transaction for the customer and original item you were attempting to use it on. Put the discount code here.
#code not working