Issue
I've provisioned Heroku Connect and I'm trying to authorize it to access my sandbox org, but I'm unable to login to Salesforce using my production credentials.
Resolution
When a sandbox org is created, it copies the configured users over from the production environment along with the rest of your data, but Salesforce usernames are unique across all environment, which requires that they change for the sandbox environment. When you create a sandbox, you're asked to give it a short name (for example, test), and that name gets appended to the end of the usernames you already have configured. So username@example.com in production would be username@example.com.test when copied into the test sandbox.
Reference: https://help.salesforce.com/articleView?id=data_sandbox_create.htm#sandbox_modified_username