I'm very lost with the gateway. It seems that Authorize.net is highly supported with Craft Commerce but the gateway isn't working. I had the dummy gateway working perfectly fine so that when I'd go through the whole processes and the order would show up. I put in authorize.net AIM as a new gateway and now when I get to the payment it does nothing at all. I put in this code

{% if craft.config.devMode and (craft.session.hasFlash('notice') or craft.session.hasFlash('error')) %}

  {% set noticeFlash = craft.session.getFlash('notice')|join('|') %}
  {% set errorFlash = craft.session.getFlash('error')|join('|') %}

    Notice: [{{  noticeFlash }}]
    Error: [{{  errorFlash }}]

 {% endif %}

and I get back

Error: [Payment method does not exist or is not allowed.]

I've put in the API user and key and the link at the bottom. Am I supposed to set the gateway as default somewhere?