paypal gateway not working properly

Hi,
on my WPGeodirectory site I have installed the WPinvoicing plugin and activated two payment gateways:

paypal-standard
bank transfer

(see screenshot ‘active gateway’)

There seems to be a problem with the paypal-standard gateway, even though I have entered my relevant email address in the field required (see screenshot ‘paypal gateway’)

Instead of being forwarded to his/her paypal account, users receive a failure notice “return to merchant” (see screenshot ‘paypal return to merchant’).

To get this going, do I need to activate any of theese addons? (see screenshot ‘addons’)
*2Checkout Payment Gateway
*PayFast Payment Gateway
*Sage Pay Payment Gateway
*Stripe Payment Gateway

If yes, which one? If no, what else must I do?

Many thanks in advance for your support!
Birgit