There are lots of reasons why a message might fail to send. If the receiving node is an older version of LND (before 0.9.0), not running with --accept-keysend flag, or a node that does not support keysend payments then you will not be able to message that node. If there is no route available to reach the destination then you will need to open new channels to be able to reach them. It's also possible nodes along the route are charging high fees to route your messages. In this case you can increment the fee limit you are willing to pay.