There are limits to automatic configuration. If you install redmine-pgsql and configure redmine to use postgres,
you are supposed to know what postgres is, and how to make sure you have a running postgres server somewhere.
Since it seems to not be your case, you'd better install
redmine-sqlite and configure it with sqlite database. It is simpler and doesn't require any other installation/configuration.
There are limits to automatic configuration. If you install redmine-pgsql and configure redmine to use postgres, configuration.
you are supposed to know what postgres is, and how to make sure you have a running postgres server somewhere.
Since it seems to not be your case, you'd better install
redmine-sqlite and configure it with sqlite database. It is simpler and doesn't require any other installation/