Issue
You are seeing the error:
There is no scheduled maintenance for this database
When trying to run maintenance on your Heroku Postgres Database
Resolution
This means there is currently no maintenance necessary for your database so Heroku has not automatically scheduled one for you based on your maintenance window.
You can create new maintenances yourself using the schedule command of the Data Maintenances CLI.
Data Maintenance CLI Installation details here
You can also use the schedule command to reschedule an existing maintenance.