Check it is not running first.
ps aux|grep fail2banIf it has started up automatically following installation you should see something like:
root 1105 0.1 0.1 300736 22632 ? Ssl 06:40 0:14 /usr/bin/python3 \
/usr/bin/fail2ban-server -xf start
If it has not started running automatically:
sudo systemctl start fail2ban