If you are still getting the error  "Customer information does not meet the requirements for the XXXXXXX discount code"  after you entered your email address and submitted again it may be because the code is only applicable for first-time customers.  

Or, are you having trouble finding the spot for entering a promo code?  It's not obvious on mobile (sorry about that!).   You need to click on the "Show Order Summary and Enter Promo Code" box to expand the section.