ok so in nano paste the following line in

* * * * * cd /home/YOURUSERNAME/sentinel && ./venv/bin/python bin/sentinel.py >/dev/null 2>&1


where it says YOURUSERNAME change this to the name of the user you made (in the monacocoin website tutorial they use "bob")

so like this

* * * * * cd /home/bob/sentinel && ./venv/bin/python bin/sentinel.py >/dev/null 2>&1