Spotlight built-in calculator does not work

I was having the same problem as you do and I notice that the problem was that I have conversion disabled on System Preferentes

So I went to System Preferences > Spotlight, enabled it and it solved my problem.

I have done the way suggested by @portella 5 years ago, but it does not solve my problem. I am using macOS Catalina (ver 10.15.14), and so it is newer than that where the suggestion worked. I guess that the issue in Catalina comes from a different cause.

After trial and error, which is just checking-in and checking-out boxes in preferences of Spotlight, I found that at least for my case, enabling "Applications" works, somehow. I guess that because in this version, calculator is recognized as one of applications, using calculator in spotlight requires permission of using applications.

In short, for macOS Catalina, enabling System Preferences -> Spotlight -> Applications may solve the issue.

Enabling Applications in System Preferences -> Spotlight -> Applications solved the issue for me.

The manual way is you tell Spotlight to exclude your drive—which erases the index—and then removing it from exclusion tells Spotlight to go ahead and index it again.

go to System Preferences > Siri & Spotlight > Spotlight Privacy

add Macintosh HD (or whatever drive you want to reindex)

click Done

immediately return to that dialogue and remove Macintosh HD