In order to enable People Insights on your system, you will first need to see the Background Processes menu, which might be hidden by default.
Follow the steps below to display the menu and enable the setting.
Show Background Processes
Go to Setup, then Settings.
Click Menu Structures, then AccessHR.
Scroll down to Other and click Background Processes.
From the Menu Group dropdown, select Setup.
Click Save.
Go to Setup, then Roles & Permissions.
Click the System Owner role.
Search for Background Processes Menu and click the eye icon so that it's green.
Click Save.
Enable People Insights Hub
Go to Setup, then Background Processes.
Search for PeopleInsights - Absence Cache Refresh.
Click the toggle icon to set it to ON.