When searching Outlook on a macOS device, if you are not returning any results please try this:
To fix this issue you must turn on the New Outlook by toggling the New Outlook switch located in the top right of the Outlook application window to ON. Once turned ON, searching should begin working within the Outlook application.
If you cannot locate the New Outlook switch, select Help followed by Check for updates and make sure the Outlook application is up to date. If it is not, update the application. Once updated the New Outlook switch should appear for you to turn ON.