App Store requires built-in Ethernet or WiFi at 'en0'.
USB WiFi/USB tethering cannot be made built-in.
If your laptop has Ethernet, install Ethernet drivers... make sure it is assigned to 'en0'
If no Ethernet, install supported PCIe WiFi... make sure it is assigned to 'en0'.
If no supported WiFi, install NullEthernet
See FAQ:
http://www.tonymacx86.com/el-capita...faq-read-first-laptop-frequent-questions.html