Could you please add the following to the
Subsystem sftp /usr/lib/openssh/sftp-server -l INFO
Then, restar the ssd server with
sudo service sshd restart.
/var/auth.log file will display new information each time an user is connecting through SFTP to your server. Instead of info, you can try configuring one of these log levels:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3
With that, you should be able to log the SFTP connection of your user and we could debug this. It also would be helpful if you share with us the tutorial you are following.