Using a link to preview a resume - for example from the Candidate or Application records - invokes a file previewer. The original Adobe Flash based previewer was replaced in the Sage People Spring 21 R1 Release (November 2020) with one based on the Salesforce Lightning File Previewer.
The new file previewer requires My Domain to be setup for your org. Most Sage People orgs have My Domain configured and Salesforce production orgs created in the Salesforce Winter 21 release and later have a My Domain by default. As an Administrator, if My Domain has not been setup you can follow Salesforce guidance to set up My Domain for your org.
If My Domain is not setup for your org and you try to preview a resume file, Sage People displays:
Lightning components require My Domain. Please contact your System Administrator.