Using 16 digit Mastercard and Visa glitch

Credit cards numbers in Europe consist of 16-digits too, so I’m not sure why you would think that the 16 digits are the problem. Also as you write, you’ve successfully used your credit card in various countries in Europe, so that can’t be the issue.

Edited to add; I see you already have another topic about this issue
https://community.ricksteves.com/travel-forum/greece/book-a-ferry-ticket-on-hellenic-seaways

American Express cards have 15 digits, and yet they’re accepted in fewer places in Europe than VISA and MasterCard. I wonder if the ferry’s transaction processing system is temporarily malfunctioning? Or is it rejecting foreign customers for some reason? Are cookies on that Website preventing successful purchasing?

Or is the bank that issued your credit card blocking the transaction for some reason? Maybe call the toll-free customer service number on the back of your card(s), and ensure that they’re not the reason the purchase isn’t going through. If they’re responsible, you should be able to clear that up with them, and have them b allow the purchase.

Have a look here for one possible explanation:

3-D Secure credit card payments - wha???

What is the nature of the failure? It may be a 3DSecure failure. If that is the case my understanding is that your bank or card issuer will not see any activity. The other possibility is your bank actively blocks certain countries. Are you traveling now or are you making purchases from the US. Have you made other purchases from Greek vendors? You say that you have not had issues with France, Spain, Prague, and Italy but you are with Hellenic Seaways. Your bank may have a country-level geoblock on Greece. Talk to you bank.

https://wallester.com/blog/business-insights/3d-secure-authentication-failed-causes-and-fixes

Have you contacted the card issuer to see if/why they may be blocking the transaction? Or see if they can advise what the solution is to whatever the problem is?

When we were purchasing train tickets with vy.no for Norway several years ago, their website stated that most North American credit cards would not be accepted due to lack of security features. We had to use PayPal to purchase our tickets. Once we were in Norway our US credit cards worked flawlessly.