Getting Error Message from payment gateway
I did a workaround when i got this error:


Edit the Contribution page
Go to "Amounts"
click "Test Processor"
Save


I assume a prior processor wasn't showing up or configured after i reinstalled my server and re-enabled Drupal and CiviCRM. 

By selecting a different processor, it fixes the problem. If you really need the original, i'm sure you can recreate the payment processor and set it.