Most answer here are rather heavy-handed. To free some disk space you may want to archive or delete this.

~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/ will contain backups of your previous database.In my case this took about 2–3 hours on a speedy MacBook Pro.

Outlook is now forced to restore its database and rebuild the index, etc. to the Desktop (to keep it around until the restore is successful) In my case, I finally found that ' alternative method 6' from that guide worked. This website seems to have the most comprehensive guides for troubleshooting this. With Mojave & Outlook 2019, however, I had to try many other methods. With previous version I had success with the Repair tool that you mention. Outlook and macOS's Spotlight (the indexing app) have notorious problems working together.