Ascertain that your Outlook profile isn’t listed under Spotlight’s Privacy tab
This section will shine the light on the best-practice techniques to make Outlook for Mac work the way it should. If you can manually find a specific message in your mail folders but Outlook keeps displaying the obnoxious “No Results” error when you use its Search box to look for stuff, the following tips will point you in the right direction.
The root cause for this predicament varies, and therefore the fix is usually a matter of trial and error. In the unlikely event the Spotlight index is corrupted, you may have to go the extra mile getting the feature to re-index messages. One of the possible reasons is that Spotlight hasn’t yet indexed your data if you have recently added a new profile to Outlook for Mac.
It’s hard to say why exactly this adverse situation might occur, but it makes some emails slip under the radar when you run a search. If so, the Mac’s Spotlight feature is blocked from searching that location.