Error whilst trying to retrieve Auth Token

Hello,

When trying to retrieve my Auth Token using the curl command as per your email, I'm getting the following error:

{"error":"access_denied","error_description":"Client is not authorized to access "$client". You need to create a "client-grant" associated to this API. See: https://auth0.com/docs/api/v2#!/Client_Grants/post_client_grants"}%

Could you help urgently with this please?

Many thanks
Phil