Debian's system crontab log needs to be opened manually,

Find directory /etc/rsyslog.conf ,

Remove About cron The comment on that line # Remove,

 # cron.*                          /var/log/cron.log

restart debian Systematic log Services,

 /etc/init.d/rsyslog restart

reference resources:

https://yesnotes.wordpress.com/2013/05/11/debian-crontab-opening-logging/