"The creditor may not be supplied when the type is DebtorPayment" really says it: Since the entry you're creating is a DebtorPayment, you cannot specify a Creditor - it doesn't make any sense.
Similarly, neither does setting the Account (in practice, the Account of a DebtorPayment is automatically set to the debtor's group's account).
Out of Account, ContraAccount, Debtor and Creditor, for each entry type, these are the ones you can set (and for each, you must set at least one):
ManualDebtorInvoice/DebtorPayment: Debtor and/or ContraAccount
CreditorInvoice/CreditorPayment: Creditor and/or ContraAccount
FinanceVoucher: Account and/or ContraAccount