What is a possible reason for the error message?
Router(config)#aaa server?% Unrecognized command

A.
The command syntax requires a space after the word “server”

B.
The command is invalid on the target device

C.
The router is already running the latest operating system

D.
The router is a new device on which the aaa new-model command must be applied before continuing

Explanation:
BD
Before you can use any of the services AAA network security services provide, you must enable AAA.
Source: http://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfaaa.html