Paypal Sandbox Issue

I'm having issues getting Paypal Sandbox to work on MarketPress.
I've alredy do this:
- Logged into my Paypal Sandbox Account.
- Set up Paypal Credentials on MarketPress and turned on Sandbox mode on settings.
- Created a Preconfigured Buyer and Seller test account.

All the process went ok. But when I do the checkout I have this message:
We cannot process this transaction because there is a problem with the PayPal email address supplied by the seller. (I've attached a screenshot)

Any Ideas what I'm doing wrong?

Thanks guys