Mac white screen with no entry sign

[bruce1478]

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

 

[Just_Me_D]

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

 

[bruce1478]

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