heroku pg:psql not working

As started to hint already using DATABASE_URL there are few more things to consider.

If you have multiple apps in Heroku it is always good to specify to what app you want to connect to where DB is added. Same applies if you have multiple databases to specify DB name (if it is something else than default DATABASE_URL).

Some examples how to connect:

-- Defining to what DB and app you want to connect to:
heroku pg:psql [lower case generated name] -a myapp
heroku pg:psql [lower case generated name] --app myapp
heroku pg:psql HEROKU_POSTGRESQL_[upper case generated name]_URL -a myapp

-- This will fall back to default DATABASE_URL part of your app:
heroku pg:psql -a myapp

Also to keep in mind as said here https://devcenter.heroku.com/articles/heroku-local#copy-heroku-config-vars-to-your-local-env-file:

Keep in mind that your deployed production app may be connecting to different services than your local development app. For example, your deployed production app might have a DATABASE_URL config var that references a Heroku Postgres database, but your local app might have a DATABASE_URL variable in the .env file that references your local installation of Postgres.

Heroku guideline – https://devcenter.heroku.com/articles/heroku-postgresql#pg-psql (but it explains little about multi-database and multi-apps cases).

Heroku guideline for connecting externally - https://devcenter.heroku.com/articles/connecting-to-heroku-postgres-databases-from-outside-of-heroku