Upgraded to El Capitan - now I'm unable to login to app store or iTunes
From this computer (MacBook Air 10.11.2, 1.3Gz, 4GB DDR3) I am unable to login to anything that requires an Apple ID. Mainly the pain point is I can't get updates or download apps from the App Store and I can't use iTunes.
This happened after I upgraded to El Capitan. How can I resolve this or has anyone had a similar problem.
Thanks
MacBook Air, OS X El Capitan (10.11.2)