Each Space has its own configuration so that specific settings can be applied by Space Admins.
To enable page classification in a space on Confluence, access the Space Settings in the left hand navigation bar.
Click on Content Tools on the top tab bar, then on Compliance, as shown in the screenshot below.
This will bring you to the configuration screen where you can enable/disable classification and set whether to enable the Automatic Page Restriction. Click the button Enable Compliance to toggle the page classification for this Space.