You've accidentally changed your database credentials with
heroku rollback or by directly editing a config var. You need to ensure your config vars have correct values that will get updated automatically in the future.
You can verify your current credentials running
heroku pg:credentials:url -a <app_name>.
You can also reset your credentials with the
heroku pg:credentials:rotate command. That will create a new username and password for your database and update the associated config variable. If you have manually changed other config vars on this or another app, you can use the
heroku addons:attach command to attach your database addon to the same app, or to another Heroku app.