Comment 1 for bug 1786967

juju-core relates to juju 1.X.
I'm guessing you mean to target 2.x.

Zone is not allowed as a constraint (--constraints), it is allowed as a placement directive (--to). The difference is that the former is applied to all units of an application (so juju add-unit X would also target exactly that zone), while --to applies only to the next unit that is being added.
Zone is not currently allowed as a constraint because it conflicts with the idea of availability. (If zones are defined as availability
zones as they are in most places, then zone on an application prevents you from actually giving good availability when you have more than one unit.)
I don't know if you could put a "zone" into the "to:" section of an application, but it doesn't fit under constraints.