Check out not working

I get a message saying my HSN card has been declined, despite the fact my limit is far higher than my current balance. I tried using a different credit card, but I get a message saying that the $20 discount isn’t valid with my Visa. I tried calling customer service, but it is closed.
I’m concerned that my items will sell out before I can get the problem resolved.

I noticed a post where someone else had a similar problem two days ago.