SQL server AlwaysOn database not automatically seeding, cannot find logs with reason
On the secondary replica, you should be able to see the status / outcome of seeding attempts in the sys.dm_hadr_automatic_seeding DMV.  This includes columns like current_state, failure_state, and failure_state_desc, as well as timing related columns, that should give you a decent overview of seeding issues.

I have often seen seeding errors written to the SQL Server error log (also on the secondary replica), I don't think you mentioned looking there:



One thing I often forget to do is to run this on the secondary replica:

ALTER AVAILABILITY GROUP yourAgName GRANT CREATE ANY DATABASE;


Note: if you're interested in seeing which replicas have automatic seeding permissions, check out this blog post from Sean Gallardy: Automatic Seeding – Which Replicas Have Permission?

Once you've done that, you can attempt re-seeding by running this on the primary:

ALTER AVAILABILITY GROUP yourAgName
    MODIFY REPLICA ON 'SecondaryReplicaInstanceName' 
    WITH (SEEDING_MODE = AUTOMATIC);


That normally gets me up and running.



As far as this question:


  Is there a canonical guide to ALL the places one is supposed to look for AlwaysOn related errors?


There is a great blog post from the SQL Server team here: Failover Detection Utility – Availability Group Failover Analysis Made Easy

It's about troubleshooting failovers specifically, but it lists out the main places where you'll see AG errors and diagnostic information:


SQL error logs
Always On Availability Groups Extended Event Logs
System Health Extended Event Logs
System log
Windows cluster log