Approving an EMV transaction is actually a two-step process. Both the card issuer and the chip itself must approve the transaction. First, the payment request is sent to the card issuer. Once the card issuer decides whether or not to approve the transaction, it sends its decision back to the chip. The chip then has to make the final determination and finalize the transaction. The card needs to stay inserted throughout the entirety of this process.
This error means that the chip made the decision to decline the transaction.
This could have been caused by a miscommunication between the card issuer and the chip, usually due to the authorization data being presented in a format that couldn't be read by the chip.
Alternatively, the chip may have detected that the card was being used fraudulently and declined the transaction for that reason. Ask the customer for another method of payment.