Way out of the war I offered to join?
Solution 1:
This is likely a bug in 2.2 - see the notes for the next patch.
- Fixed bug where the AI did not end wars when at 100% warscore due to the recipient having previously declined a peace offer.
Certain wars are inherited, and it looks like this one is, so there's no fix other than wait for the patch or consoling over to the Aztecs and accepting peace if not ironman.