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.