Change the code table entry for credit card type to American Express.  For example, if you have something like AmEx in your code tables, instead of American Express, you will receive this error:
  1. Go to Administration>Code tables
  2. From the Revenue section, select the Credit Type code table and click Go to entry list
  3. Select the entry used for American Express credit cards and click Edit (example, AmEx, Am Ex, etc).
  4. Change the table entry to American Express
  5. Save.