Hi,
As you say, this error is a generic error message which can happens for a lot of reasons. There are many solutions possible because it can come from a lot of things.
The best here is to contact the SagePay support with the debug data of your screenshot and a screenshot of your payment method settings. They should be able to check what's wrong based on how your merchant account is configured, which is something we can't do on our end.
For example, it could be that the mode that you selected in the payment method doesn't correspond to the mode that you have configured in your merchant account, or the vendor name is different, etc.