This is related to PIN validation, and is only encountered when using PIN-protected transactions.
When an incorrect PIN is provided three consecutive times for a PIN-protected transaction, the card will become locked.
When a card is locked, it cannot be used for any further PIN-protected transactions until it is unlocked. A locked card can be unlocked via the Card Status page in Connect Manager, or via the ‘Unlock’ endpoint
A transaction that fails due to a card being locked will return response code 16.