Sage Pay Form (or Sage Pay Go form integration) is an online payments processor working in UK. You will find more details on the Sage Pay site: www.sagepay.com/sage-pay-form-integration
If you want to set up Sage Exchange (Sage payment gateway for US), please refer to this article: Sage Exchange
In order to enable and use Sage Pay Form as a payment option, please follow these steps:
1. Sign in to your Online store control panel.
2. Go to the Settings → Payment page. Change the Payment Processor value of the "Credit Card" payment method to "SagePay Form". Save the changes.
3. Click the [Account Details] link near the SagePay payment method. Set your Vendor Name and Encryption password.
Vendor Name is your unique company identifier, assigned to you by SagePay when your account was created.
Encryption password is an encryption password assigned to you by SagePay (not the password to login to your SagePay merchant interface). You should have received encryption password by email from Sage Pay when your Sage Pay account was created.
4. Save the changes and enable this payment method.
The error indicates that the currency selected in your control panel differs from the currency defined in your SagePay account. Troubleshooting steps:
1. Check what currency is chosen on Settings → General → Regional Settings page of control panel.
2. Sign in to your SagePay account and make sure it has the same currency that is chosen in the control panel.