Happened same with me. This is happening because the App Store is looking for a working connection on en0 port and it is not getting anything anything on it due to which it says the device cannot be verified.
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Your task is to switch the en0 and en1 assignments basically.
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Just copy the files 'preferences.plist' and 'NetworkInterfaces.plist' from
/Library/Preferences/SystemConfigurationto Desktop as a backup and delete it from/Library/Preferences/SystemConfiguration -
Restart your Mac and try to login into the App Store
Worked for me!