If you are getting this error, error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
This is because you are on a Mac, and you upgraded to macOS Sierra.
To fix this, all you need to do is update/install Xcode.
run this command in the terminal:
Run the command, agree to the agreement and install Xcode.
This will fix the issue.