here is the summary of the solution:
The user does not exist in the connection you are trying to connect to hence getting the wrong username or password.
As a workaround, you can try going to the Connections tab in the application and selecting the connection.
To provide more information, the Resource Owner Password Flow uses /token endpoint directly. other flows that use /authorize you may as well use the connection parameter to force the connection you want to connect to (not in the flow you are using), in the authentication API you can see the allowed parameters for the different flows.