Refreshing an access token

[lwb4]

I tried to follow auth0's tutorial for refreshing my access token, but I get the following error in Postman:

{
    "error": "invalid_grant",
    "message": "The provided authorization grant (e.g., authorization code, resource owner credentials) or refresh token is invalid, expired, revoked, does not match the redirection URI used in the authorization request, or was issued to another client.",
    "hint": "Check the configuration to see if the grant is enabled."
}

This is a screenshot of my Postman setup:

Is there some configuration I have to do on the backend to support refresh tokens?