You have something known as DATABASE_URL, which has to be used in the backup command. To find your database url, run the following command:

heroku pg
=== HEROKU_POSTGRESQL_JUPITER_URL
Plan:                  Hobby-dev
Status:                Available
Connections:           1/20
PG Version:            10.15
Created:               2018-12-16 08:57 UTC
Data Size:             12.9 MB
Tables:                39
Rows:                  4751/10000 (In compliance)
Fork/Follow:           Unsupported
Rollback:              Unsupported
Continuous Protection: Off
Add-on:                postgresql-tapered-53472

Now run the command to create the backup:

$ heroku pg:backups:capture HEROKU_POSTGRESQL_JUPITER_URL
Starting backup of postgresql-copred-72... done

Use Ctrl-C at any time to stop monitoring progress; the backup will continue running.
Use heroku pg:backups:info to check progress.
Stop a running backup with heroku pg:backups:cancel.

Backing up JUPITER to b007... done

Now, download the backup to local folder:

$ heroku pg:backups:download
Getting backup from ⬢ mysterious-boco-302564... done, #7
Downloading latest.dump... ████████████████████████▏  100% 00:00 291.55KB 

The backup has been created in the current directory with the name latest.dump.

Reference: Heroku doku

To restore a backup, use the restore command:

$ heroku pg:backups:restore b101 DATABASE_URL --app sushi

This will restore backup id b101 to the specified database URL in the app sushi. Note: you can omit the backup id and the target database to restore the latest backup to DATABASE_URL, otherwise both backup id and target database must be provided.