Password Grant Error: Cannot read property 'scope' of undefined

When I try password grant either via postman or auth0 api debugger I get the error:

    "error": "access_denied",
    "error_description": "Cannot read property 'scope' of undefined"

The most common cause for an access_denied error where the description indicates some sort of Javascript error is the use of a custom rule that is triggering that code error and as a consequence invalidating the authentication transaction.

Review your existing rules and update them in order to not cause error during the authentication transaction.

Brilliant. That was it!