When navigating to the Detect -> Users section, there is no Inactivity option.
The corresponding detector is not enabled.
Go to the Preferences section of the Detect module then Incident Detectors. Enable the Inactivity Period Detector and select the threshold for this detector as shown below:
Be sure to 'Save Changes' at the bottom of this page.
For additional information refer to the Viewing user inactivity in the CASB Tech Docs.