Cannot change permissions on /System/Library
You'll need to disable System Integrity Protection to make changes to permissions in /System.


  To disable System Integrity Protection, you can do the following:
  
  
  Boot into the Recovery HD by restarting whilst holding ⌘R.
  Open Terminal (from the Utilities menu).
  Run the following command in Terminal:

csrutil disable

  Restart.
  


Disable OS X El Capitan Rootless and permit write actions to System Integrity Protection locations