Authorize.net Getting error "A valid amount is required."
I got the solution of this issue.

When we were working in local we have removed orders from CMS before moving database to Dev site. But because of dependency some orders were not removed completely. When Authorize.net finds such orders it has treating them as pending order and was adding payment in new order as Unknown payment method.


Some how this was adding extra amounts to final payment of new orders and was creating negative amount.

To resolve this I have deleted all my dummy orders from CMS and also from Table "commerce_payment_transaction".

This has resolved my issue.
One of the Drupal's Stackexchange answer helped me to resolve this issue, Why would just adding a product to cart create a payment with method type &quot;unknown&quot; and credit card of &quot;4111--------1111&quot;