If System Settings shows “No Results” for every search term after updating to macOS 27, note how many times the Mac has been restarted since the upgrade, then restart it once more and test the search.
Also check System Settings >> General >> Software Update and install any available update. Apple Support — Update macOS on Mac
Because Spotlight indexes system settings and indexing occurs after a macOS upgrade, allow it sufficient time to complete before rebuilding the index.
Apple notes that indexing can take hours or even days, depending on the amount of data. Check System Settings >> Spotlight to make sure the relevant search categories are enabled. Apple Support — About Spotlight indexing and search results
If the problem persists, rebuild the Spotlight index using Apple's procedure: open Spotlight >> Search Privacy, add the startup disk, normally Macintosh HD, wait a few seconds, remove it, and click Done.
Then allow indexing to complete before testing System Settings search again. Apple Support — About Spotlight indexing and search results
Next, start the Mac in Safe Mode, test the System Settings search, and then restart normally and test it again.
If the problem remains, create a temporary standard user account in System Settings >> Users & Groups and search for something such as Wallpaper.
If search works in the temporary account but not the original account, something associated with the original account may be involved.
If it fails in both accounts, the problem is also reproducible outside the original account.
These results should provide a clearer basis for deciding what further troubleshooting is appropriate.