MarketPress error with PayPal Standard: 520003 – Authentication failed. API credentials are incorrec

I have MarketPress, it is configured and looks great with some new shortcodes and the products listed. Thanks!

I have set up Google Checkout successfully, however PayPal standard is not configuring properly for some reason.

For PayPal standard, the chained set up should only require my PayPal user name (email address) and be set to live (it is). PayPal standard is configured on PayPal, I believe. I have not seen anything on their side to use sandbox with just the email credentials. Being that the API version is neither available to me via PayPal and not set up in MarketPress, why would I be getting the error: ‘520003 – Authentication failed. API credentials are incorrect.’?

Thanks so much,