Command Line Tools not working - OS X El Capitan, Sierra, High Sierra, Mojave

If you have issues with the xcode-select --install command; e.g. I kept getting a network problem timeout, then try downloading the dmg at developer.apple.com/downloads (Command line tools OS X 10.11) for Xcode 7.1

In macOS 10.14 this issue may also occur if you have two or more versions installed. If you like xCode GUI you can do it by going into preferences - CMD + ,, selecting Locations tab and choosing version of Command Line Tools. Please refer to the attached print screen.

Found the fix for the problem here.

xcode-select --install

This will bring up a prompt to install the necessary command line tools.