Authorize net problem - card not entered correctly

We're setting up AIM and can't get past step 2 on zen cart's checkout process.

Still in test mode, we've used the numbers supplied on the module AND used my own CC number. Nada.

Error: The credit card number starting with 6011 was not entered correctly, or we do not accept that kind of card. Please try again or use another credit card.

What are we doing wrong?

Problem solved.

For others getting the error: under the *config *tab is credit card. You must turn on which cards you'll accept.

Lovely:

Problem solved.

For others getting the error: under the *config *tab is credit card. You must turn on which cards you'll accept.

Thanks for posting this information! I had previously enabled Amex but AuthorizeNet was not allowing it as our merchant/gateway account was not configured to allow Amex, so I disabled Amex on our site until AuthorizeNet could fix their side. Once they did I forgot to enable the Amex under config and was getting this error.
Hope this info helps other people too. Thanks again!!