Invalid resource owner credentials using the API Explorer

I created a new app on dev to access messages and have the credentials provided, however when I try to authenticate in the API Explorer I get the following error:

{
  "error": "invalid_grant",
  "errors": [
    {
      "errorCode": "OAU-140",
      "message": "Invalid resource owner credentials"
    }
  ],
  "error_description": "Invalid resource owner credentials"
}