"Do not honour" is one of the most common reason messages returned for unsuccessful transactions. To merchants, it is not transparent exactly why a payment request was declined.
Issuers return a "Do not honour" response in a variety of circumstances. It is possible that the issuing bank does not have a code that fits the decline reason precisely and uses "do not honour" as a default option. In situations where a credentials check fails (such as mismatching AVS or CVC data), or the card usage pattern suggests the possibility of fraud (e.g. repeated transaction attempts within a short period), the issuing bank may not wish to disclose more specific information. "Do not honour" may also be returned in case of a failed 3D Secure check.
If the "Do not honour" response still occurs after the cardholder has verified that the card and verification data they have entered are correct, they should be advised to contact the issuing bank directly.