Discover and MasterCard payment method doesn't work in real order simulation or test mode.

[cactusanne]

I'm attempting to do test orders on my website by using a Discover and Master card. It won't work. It works fine with Visa and AMEX. The checkout page would give the error "There Was An Issue Processing Your Payment. Try Again Or Use A Different Payment Method."  These are all with Shopify Payments.

• Manual Credit Card: In the abandoned orders, it shows error "undefined method `rate' for nil:NilClass". I can't find what the issue is with this anywhere online.
• ApplePay (with Discover or MasterCard): In the abandoned orders, same error above "undefined method `rate' for nil:NilClass".
• Test Mode: Also doesn't work.

Does anyone know why this is?