You don't tell
logrotate which file to rotate on the command line. You pass it a configuration file. So in your case,
logrotate is reading
/var/log/syslog and trying to parse it as a config file and failing (hence your errors).
If you want to rotate
/var/log/syslog it needs to be listed in a
logrotate config file somewhere, and you just run
logrotate. If it rotated recently, then
logrotate -f to force it to do it again.
Here's an example entry for
/var/log/syslog from Debian,
invoke-rc.d rsyslog reload > /dev/null
So, you need that in a file, normally either
/etc/logrotate.conf or as a file snippet in
/etc/logrotate.d assuming your
/etc/logrotate.conf points there, and then you just run
Since you're running Debian, this is probably all in place, and all you need to actually do is run
logrotate -f /etc/logrotate.conf (note, this will rotate every log currently configured in logrotate).
If you truly only want to rotate
/var/log/syslog, you will need to create a config file that just does that one log, based on the content of