If your discount code is reading as not found when you go to use it as a customer; i.e. here:
Consider the following:
Is the code valid for this class or event?
Is the code restricted to certain events only?
Has the code reached its usage limit (if one has been set)?
Is the code within its validity dates?
Are you entering the code exactly as created? (If it includes capital letters, these must be entered exactly the same.)