I just upgraded to macOS Sierra this week, and all of a sudden my Spotlight Search doesn't include Applications in the results. It'll just do web and document searches. Search in Outlook doesn't work ether.
So after some Googlin', I went to System Preferences > Spotlight, and unchecked everything under Search Results, closed the settings, reopened them, and checked only Applications. No luck.
I then went to the Privacy tab, hit the + button, and selected the Applications folder. I got this error: "Privacy List Error: The item couldn't be added or removed because of an unknown error."
How to FIX:
sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
Then I waited a few minutes and then ran:
sudo mdutil -E /Volumes/*
/:
Indexing enabled.
Now it seems to work!