Problem

Applications don't show up in Spotlight search result.

Solution

Turn off spotlight

sudo mdutil -a -i off

Unload and reload it

If your macOS System Integrity Protection (SIP) is enabled, you will see an error message looks like Operation not permitted while System Integrity Protection is engaged, so you can ignore this step.

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist

Turn on spotlight again

sudo mdutil -a -i on

If the problem was not solved, try these instructions:

1. Go to System Preferences, Select Spotlight.
2. Select the Privacy tab.
3. Click the Plus (+) button.
4. Press Command + Shift + C to reveal your hard drives (if they are not available otherwise), or simply navigate to your hard drive (most likely MacintoshHD)
5. If you have more than one hard drive, hold the Command key and click your various drives to add them to your selection.
6. Click the Choose button to add them to the Privacy list.
7. Ignore the warning.
8. Remove the hard drive by clicking the minus sign.

Now everything should be reindexed as expected.