Mac white screen with no entry sign

Please help, I am selling a 2011 mbp, I erased disk in disk utility, was asked to reinstall os with an Apple ID, I didn't know the id of person selling it to so I shut mac down, now when I restart I have the above issue and don't know what to do, tried cmd & r but no joy, tried cmd &s but again no joy, any easy to follow assistance please.
I know the mbp is working fine as I was using it earlier today all fine
Thanks in anticipation

The device is protected with the iCloud activation lock. There is NOTHING you can do without the credentials associated with the owner's account.

Sorry meant to say didn't know the id of the buyer, it is a family member.
How do I get to disk utility to install os
Thanks