It can also mean that the account has been closed or the account number has been changed by the issuer.
It can mean that the card/account was bad for some other reason.
We know that if the card has already passed 2 “check digit tests,” … one within the Order and Invoice Database before the transaction is passed to us (I gave them my check-digit formula years ago) and a second test within our software before we run the transaction. So, it is doubtful that this is because the account number was entered incorrectly by the user.
In all cases, the best thing to do would be to encourage the cardholder to call their “issuing bank.”
If the card is a gift card or some other non-standard card where there is no issuing bank to call, chances are, the card would need to be swiped (or non-swiped with bad AVS)….if it will work at all.