paymentMethod in SalesOrder

When I do a add to the SalesOrder, I want to set the payment method to PayPal. The line

<tranSales:paymentMethod internalId="7" type="calendarEvent" xsi:type="platformCore:RecordRef"/>

is giving me this error message:

asdfArray
(
    [writeResponse] => Array
        (
            [status] => Array
                (
                    [statusDetail] => Array
                        (
                            [code] => INSUFFICIENT_PERMISSION
                            [message] => You do not have permissions to set a value for element paymentMethod due to one of the following reasons: 1) The field is read-only; 2) An associated feature is disabled; 3) The field is available either when a record is created or updated, but not in both cases.
                        )

                    [!isSuccess] => false