That means you are trying to add a Virtual AP to the AP-Group MEE that is already being broadcast by that AP-Group. The Virtual AP might have a different name, but the SSID profile has an ESSID parameter that is the same as a Virtual AP that is already part of that ap-group.
It probably looks like this:
ap-Group MEE
Virtual AP X
SSID-Profile Y <------------------------- The name of the SSID profile might not be the same, but the essid parameter might:
Virtual AP MEGuest
SSID-Profile X <------------------------- The name of the SSID profile might not be the same, but the ESSID profile is the same as the one in SSID-Profile Y so it does not allow you to add it to the ap-group MEE