Zen Cart evaluates the credit card numbers you enter to see if they match the settings you have enabled in the Admin->Config->Credit Cards settings.
So, if you enter a MasterCard number, but have MasterCard turned off (in the Admin), then it will tell the customer "we don't accept that kind of card".
It doesn't matter which credit card gateway you have installed ... the same processing will reject disabled card-types before even sending to the merchant gateway.
1. Review your merchant account settings (ie: with your bank). Find out what credit card types you are allowed to process.
2. Then turn those on in Zen Cart under Admin->Configuration->Credit Cards
3. Then enable your payment module in Zen Cart.
MORE INFORMATION YOU MIGHT FIND USEFUL
When using this basic/vanilla CC module, Zen Cart does not send your credit card number to anyone for validation. It uses built-in logic to evaluate the card's validity.
Zen Cart uses the MOD10 approach (google it) combined with the list of accepted cards you have configured in Admin->Configuration->Credit Cards. If the number fails validation or is a card-type that is not accepted based on your settings, it will be rejected with the error message you mentioned.
Credit Card Test Numbers you can use:
Amex Corp#: 378734493671000
You might also note that Zen Cart uses these Credit Card Prefix Patterns (and more) to detect which kind of card your customer has entered.
Visa: 13 or 16 numbers starting with 4
MasterCard: 16 numbers starting with 5
Discover: 16 numbers starting with 6011
AMEX: 15 numbers starting with 34 or 37
There are many other articles about the anatomy of credit card numbers