The Cisco 877VAW is a newer version of the 800 series routers and as a result has the AP built in on a different plane and so you have to login to the Wireless AP to be able to configure the WiFi. You do not do this from the Router config.
Note Wireless Interface
The router has a Wireless Interface called wlan-ap0 this is the service module interface, this is linked ip unnumbered to VLAN 1
Login to Service Module
You can connect to the service module with the command service-module wlan-ap0 session
Disconnect from Service Module
Once you are connected to the service module and you want to return to the router config you need to disconnect, this is done with the commands
You can now see the familiar config for a Cisco Wireless AP using IOS.