Q: Mac App Store not signing in after wifi hardware not installed error appeared. Please help!

I Recently upgraded my El Capitan is to Sierra beta. Everything was fine on the first day and suddenly I got the error Wifi no hardware installed. I connected the Ethernet port with thunderbolt. Internet came back but Mac App Store stopped Signing in. This problem did not go even after downgrading to El Capitan.

Apple authorised service advisor suggested replacing airport card could solve problem, if not the whole logic board. I do now want to invest a huge amount now on this computer. how to solve this problem?