There are a number of possible reasons why a FriendTalk message is not delivered to a KakaoTalk user and/or returns an error:
The user has not added the channel as friends. FriendTalk messages can only be sent to user who has added the channel as friends. To be a friend of a KakaoTalk channel, go to the Channel tab and click the Add Channel button to the right of the KakaoTalk channel name:
The user has not used KakaoTalk for a while.
The user has deactivated KakaoTalk push messages.
The user does not have the minimum KakaoTalk version or unsupported device.
The message was sent after 9PM and before 8AM Korean time.