Oh, unlucky. That bug does happen sometimes.
If you have dlc that allows you to play as pagan feudal (ie. old gods or holy fury) then the easiest way would be to switch to the Aztec emperor, declare war on the best target, then switch back. This will
probably (but not definitely) make the ai active again.
- back up your save
- pause
- "charinfo"
- hover over the Aztec emperor's portrait, and write down his char id from the tooltip
- if the Aztec emperor does not have a county:
- choose a county to give to him
- "debug_mode"
- hover over the county on the map (or is it the county shield?) to get the county id (of the form "c_something")
- "give_title {aztec emperor charid} {county id}"
- "play {aztec emperor charid}"
- declare war
- Not sure if you'll have access to the invasion cb, but you should have county conquest at least
- look up your original character's charid
- "play {original character id}"
- Optional: "debug_mode" and/or "charinfo", to disable the extra info in tooltips
- unpause
If you can't play as pagan, giving the emperor a county (without declaring war) might be enough to jolt them out of inactivity.
EDIT: If you give the Aztecs a county, you'll need to watch them closely in the first war: it'll be relatively easy for them to be defeated due to 100% occupation.