Try this, but not tested
open extensions/paypal_module/models/Paypal_model.php and add the following line
$nvp_data .= '&SOLUTIONTYPE=Sole';
right before
$response = $this->callPayPal('SetExpressCheckout', $nvp_data);
You must have PayPal 'Account Optional' set to 'On' within the PayPal account settings.
Here's how:
Log in to your PayPal account.
Select Profile > Profile and settings.
Click My selling tools.
Click Update next to Website preferences.
Click On for PayPal Account Optional.
Click Save.