This isn’t a native feature of reports in Docebo; however, you could generate two reports and compare them: Viewer - Asset Details and Users.
Generate the Viewer - Asset Details report with the filters All Users and All Assets selected. Once you get the report, delete the asset titles and remove any duplicate usernames. This will give you a list of all users who have viewed at least one asset. Next, generate the Users report. This gives you a list of all users.
You could then quickly compare them using conditional formatting or something like vlookup. Essentially, if the user is in the Users report, but not in the Viewer - Asset Details report, then we can infer that the person hasn’t viewed any assets.