Coupon is not valid

Hi there,
I added a coupon, I published it. I entered the correct code but I still get the message "Code is not valid". What do I miss?
Thanks for a hint. URL: www.beziehung-partnerschaft.com/index.php/audiobook
Chris

Hi,

That coupon will only be usable if you didn't use it already (there is a quota of 1 per user) and if you have the product "Wie Sie lhren..." in your cart.

I'm able to add the coupon in your website while not logged in. So I suppose that you're logged in with a user who already used that coupon once.