System Integrity Protection(SIP) aka rootless is a new security feature in OS X 10.11. By default it disables even the administrator to access/modify the root. Follow these steps to disable SIP:
Restart your Mac.
Before OS X starts up, hold down Command-R and keep it held down until you see an Apple icon and a progress bar. Release. This boots you into Recovery.
From the Utilities menu, select Terminal.
At the prompt type exactly the following and then press Return:
csrutil disable
Terminal should display a message that SIP was disabled.
From the menu, select Restart.
You can re-enable SIP by following the above steps, but using
csrutil enable
instead.