[Heroku Connect] How to map columns from an existing PostgreSQL table to fields in a Salesforce custom object

Hi,    I am planning to use Heroku Connect for the first time in an app I am currently building.    I have an existing app database and would like to sync some tables in that database to a custom object in Salesforce via Heroku Connect.    So I am going to start sync with the prefecture table like below.    PostgreSQL: prefectures:  1. id: bigint  2. name: varchar(6)  3. created_at: timestamp  4. updated_at: timestamp    Salesforce: Prefecture__c:  1. ExternalId__c: Number(18, 0)  2. Name: Text(80)  3. CreatedDate DateTime  4. LastModifiedDate DateTime    I want to map a column from the existing PostgreSQL prefectures table to a field in Salesforce's Prefecture__c, but the mapping page does not show the PostgreSQL table list.    Articles on the web state that PostgreSQL tables can be selected from a list, but I don't see any such element.    The Salesforce integration user permissions, Heroku Postgres, and Heroku Connect plans seem to be fine, but is there something I'm missing from consideration?    I'm afraid this is a rough question, but I would appreciate any hints you can give me to help me realize what I'm missing.