It is a error when I tried to connect an FTP server by LFTP tool.
Fatal error: Certificate verification: Not trusted
I did a search on Google and find the answer.
There is self signed certificates which is used by TLS on a ftp connection. It is not trused and error comes.
Of course, the connection is closed on error, and no more further put or get command can be used.
To bypass this message, or ignore this error, add following content in a file ~/.lftp/rc
set ssl:verify-certificate no