Invalid active developer path error after upgrading to El Capitan

Try:

sudo xcode-select -r

in this way it unsets any user-specified developer directory, so that the developer directory will be found via the default search mechanism.

1)

try typing in "xcode-select -print-path" and see if the path you see is the path you're expecting for your current Xcode version.

2)

If you go to:

https://developer.apple.com/downloads/

(you'll need to sign in with your Apple Developer account)

Once there, you'll see links for "Command Line Tools". It sounds like you need to install MacOS 10.11 Command Line Tools for Xcode 7.

This is all you need to do:

$ xcode-select --install

You can select command line tools from the XCode Preferences shown in the screenshot below.

You will get a prompt for password.