On attempt to configure PayPalExpress for the reseller:
Home > Configuration Director > E-Commerce Settings > Processing Centers > Processing Center PayPalExpress Processing Center > General > Edit button
an unclear error occurs:
Error: Payment System Payment System ID cannot be empty.
The payment system is not configured for the recurring payments.
An error message is to be changed within the bug #PBA-56977 fixing.
To configure the payment system it is necessary to add the custom payment method
PayPalRecurring and then PayPalExpress processing center field becomes filled with the value: "Payment System" = PayPalRecurring".