There is no option to switch on Fail2Ban jails at Plesk > Tools & Settings > IP Address Banning (Fail2Ban).
Only the Switch Off button is present.
How to enable Fail2Ban jails?
The fact that there is no option to enable jails is that Fail2Ban is currently switched off in Plesk.
To enable Fail2Ban and jails perform the following steps:
In Plesk go to Tools & Settings > IP Address Banning (Fail2Ban) > Settings tab and click Enable intrusion detection.
Click OK to apply the changes.
Switch to the Jails tab, select required jails and click Switch On.