Hey Will - you did not state if it ever worked or not. So consider this my "dumb" question of the day to you. When you define an aggregated ethernet I/F you must first created it logically by setting the following:
user@host# set chassis aggregated-devices ethernet device-count X <where "X" equals the number you want to create>
If you do this then when you do an operational show you will then see the I/F <In a down state if no other config is done yet>. As you are getting an error on the show command I am wondering if this step was left out.