Maybe this helps... its a table that mentions AP-AP Communication for Client Policies. https://www.cisco.com/c/en/us/td/docs/wireless/controller/9800/17-8/release-notes/rn-17-8-9800.html#Cisco_Reference.dita_fd1a1a9f-282c-4d1e-8948-166cb824fb8f  

Hey!I happen to see this issue in my labOn non-root AP:*Mar 27 03:10:39.930: 36B35A3D-1 Uplink: WPAIE not found and requiredThis was because the SSID config in the root didn't have the following line:wpa-psk ascii PASSWORDOnce I added it to the root ...