Originally Posted by
RUMBEAR
My guess is the FMGC was still in Cruise Phase as you mentioned it was an early descent. Altitude CSTR cannot be entered ar or above the cruising level.
I've seen this on the Honeywell neos if you're navigating to an ABEAM point. Clear The AB using Direct To and you should be able to build the constraint.
Given enough time in cruise this monkey found an answer :-)