FWIW - I just tried to set a quick test project up and I recalled the discussion here so I thought I’d check it. Therefore I left both user and password at the default “postgres” and encountered the same issue/errors OP described in his first post. The solution in my case:

silverdr$ sudo -u postgres psql
psql (12.9 (Ubuntu 12.9-0ubuntu0.20.04.1))
Type "help" for help.

postgres=# \password postgres
Enter new password: 
Enter it again: 
postgres=# \q
silverdr$ mix ecto.create
Compiling 29 files (.ex)
Generated testapp app
The database for Testapp.Repo has been created

IOW - altering the ROLE “postgres” to have password “postgres” helped immediately. This is basically the same what @inko mentioned: if you want to use the “postgres” role, give it the password you have cofigured in the Elixir application config.

You need to check what “ROLES” your Postgres installation has available, and possibly create a suitable one. PostgreSQL docs say:

In order to bootstrap the database system, a freshly initialized system always contains one predefined role. This role is always a “superuser”, and by default (unless altered when running initdb ) it will have the same name as the operating system user that initialized the database cluster. Customarily, this role will be named postgres .

This means it is neither guaranteed for every PostgreSQL installation to have the postgres “role” nor it is guaranteed for it to have “postgres” as password, even if it exists.

I have confirmed thru \du that indeed a user postgres exists, i created (i am not sure if actually ecto was the one supposed to create) a database with the same name in the dev.exs in the config directory and logged in using the password postgress and the log in was successful.

I returned to my elixir/phoenix app and run mix ecto.create and the same error “password authentication failed for user postgres”.

Any other hint or solution is welcome.