I ran into this error when I tried any brew operation (like brew update and brew upgrade) on a Mac. I had just upgraded to macOS Big Sur 11.1 and suddenly brew broke!

$ brew update

xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools),
missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun

The fix, luckily, is pretty straight forward. Install the Xcode toolkit! Even if you had it installed before, you might have to re-register it or update it to the latest version.

$ xcode-select --install

If that doesn’t work, force it to reset. You’ll need sudo access for this one.

$ sudo xcode-select --reset

If even that fails, go to the Apple developer download section and download Xcode manually.