The last step the Salesforce Administrator must complete to create a private portal for the Loyalty Program using Experience Cloud is to activate the Experience Cloud site. Activation is the final step that makes the site accessible to users. Once the site is activated, members can access the portal, where they can engage with the Loyalty Program, view their points and tiers, and take advantage of program benefits. Activation is crucial as it transitions the site from a development or staging phase to a live environment.