Unable to add unified AP to a group in Cisco Prime

hello , 

i have

in HQ : Cisco Prime version 2.1  and WLC 5508  

in remotes sites  : APs 

i'm able to add WLC to a user defined group. but  not the APs.    i want to group APs by site  but the "add to group" button is not active.

there is something to do for Lightweight Access point ?