If your discount code is reading as invalid when you go to use it as a customer; i.e. here:
Consider the following:
Is this code valid for this class?
Is the code within its validity date?
Did you set the code live more than 15 minutes ago?
Are there any spaces before or after the code?
Have you already used this code with the email you are currently using? (Codes can only be used once per customer).