There must be at least one client mode profile configured before you can define a LAN-side AP radio for client mode. See Configure a Client Mode Profile.
You can set a radio on some AP models to client mode, which allows the AP to connect to existing open and PSK wireless networks, including third-party networks as a generic BYOD client. The method described here is recommended because it is often the fastest way to perform this task.
This task is part of the network policy configuration workflow. Use this task to configure a Client Mode Profile as part of an SSID for a network policy.
For more information, see About SSID Usage in Standard Wireless Networks.