Was the Clone pair in a Copied state when you attempted to execute the SRDF createpair? You'll have to wait until the Clone copy is completed before you create the SRDF relationship.
I ahve clone session activated on two devices and I am trying to do SRDF from the cone sessions's target and I am getting error that an SRDF mirror can not be added to a device in a timefinder clone session.
SRDF of Target of clone session