You are attempting to use a third-party tool, such as Amazon DMS, to replicate your Heroku Postgres database to another, non-Heroku database.
Third-party tools, such as Amazon DMS, may require
REPLICATION roles to be set on a user accessing the database. Heroku Postgres does not give customers access to the superuser role, or allow customers to grant other users the replication role, due to these roles being very privileged.
Therefore, it is not possible to use these third-party tools to replicate your Heroku Postgres database to a non-Heroku database.