The error "Transaction has been declined" is normally returned by the processor (Authorize.Net).

You may have to log into your processor (Authorize.Net) to review the details of the transaction:
More info here: https://support.authorize.net/knowledgebase/Knowledgearticle/?code=000001554
To search for a transaction:
  1. Log into Authorize.Net
  2. Click Transaction Search from the main toolbar.
  3. Enter the applicable search criteria to look up the transaction.
  4. Click Search at the bottom of the page.
  5. Click the appropriate Transaction ID to review the transaction details.