MarketPress error with Simplify Commerce

I have installed MarketPress on a site. We're using SimplifyCommerce for payment processing. I have the public and private keys in the payment settings and am using the Force SSL option.

I created a test product for 1.00 with 1.00 shipping.

When I try to submit the payment, i receive an error "There was an error processing your card: "incorrecly formatted JWS payload". Please go back and try again."

A screenshot is attached.

Thank you,

Jenny

PS – There is also a misspelling in the error text.