How to fix Outlook for Mac search
Outlook and macOS's Spotlight (the indexing app) have notorious problems working together. With previous version I had success with the Repair tool that you mention. With Mojave &amp; Outlook 2019, however, I had to try many other methods. This website seems to have the most comprehensive guides for troubleshooting this.

In my case, I finally found that "alternative method 6" from that guide worked. That means:


Backup!
Quit Outlook
Go to the folder ~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/Main Profile/Data (in Finder you can press cmd+shift+G to get there)
Move the Outlook.sqllite file e.g. to the Desktop (to keep it around until the restore is successful)
Launch Outlook. 
Outlook will prompt you to repair your database. Click "Repair". Outlook is now forced to restore its database and rebuild the index, etc. In my case this took about 2–3 hours on a speedy MacBook Pro.
~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/ will contain backups of your previous database. To free some disk space you may want to archive or delete this.