Errored messages can be found in the Error queue. If the error failed due to a problem with the receiving system being down. First confirm with the receiving system that it is back up the click either the Resolve or Resolve All links in the Error tab. For example if the system at the other end of the FTP destination node in the screen shot was powered off you would resolve them to reporcess them when it came back up.
Caution: messages in the error queue may have been rejected by the recieving system via an error acknowledgement and need to be corrected before reprocessing or possibly discarded. This requires more thought as what is in the Error queue.
You might also be interested in this question: https://www.qvera.com/kb/index.php/417/configure-destination-node-retry-sending-fails-first-attempt?show=418#a418
