Thank you for posting in Auth0 Community @justinboyd!
Can you please share what kind of application are you using? Are you following any Auth0 SDK?
As outlined in the following documentation, the ROPG flow is not recommended to be used when securing a CLI.https://auth0.com/docs/integrations/using-auth0-to-secure-a-cli
To clarify, are the username and passwords associated with an end user, or are they an identifier for a service?