Subquery returned more than 1 value

Question

Can someone help me solve the error of my query?

UPDATE [dbo].[fin_PaymentSchedule_Master]
                       SET
                             ActualPaymentDate_ColS = GETDATE()
                            ,[PaymentVoucher_ColT] = 'PV-200780'
                            ,pyLAK_ColU =
                                (
                                    CASE
                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'LAK'
                                        THEN
                                            (SELECT SUM(AmountPaid + Vat) FROM fin_PaymentVoucher_Details WHERE AP_Number = 'AP19-3131')

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'THB'
                                        THEN 0

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'USD'
                                        THEN 0
                                    END
                                )
                            ,pyTHB_ColV =
                                (
                                    CASE
                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'THB'
                                        THEN
                                            (SELECT SUM(CurAmtPaid) FROM fin_PaymentVoucher_Details WHERE AP_Number = 'AP19-3131')

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'LAK'
                                        THEN 0

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'USD'
                                        THEN 0
                                    END
                                )

                                ,pyUSD_ColW =
                                (
                                    CASE
                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'USD'
                                        THEN
                                            (SELECT SUM(CurAmtPaid) FROM fin_PaymentVoucher_Details WHERE AP_Number = 'AP19-3131')

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'LAK'
                                        THEN 0

                                        WHEN
                                            (SELECT TOP(1) CurrencyPayment FROM fin_PaymentVoucher_Header WHERE PV_No = 'PV-200780' ORDER BY Id DESC) = 'THB'
                                        THEN 0
                                    END
                                )
                            ,pyTotalLAK_ColX = (SELECT SUM(AmountPaid + Vat) FROM fin_PaymentVoucher_Details WHERE AP_Number = 'AP19-3131')

                        WHERE AP_Number_ColA = 'AP19-3131'

Error says:

 

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

The statement has been terminated.  The code snipe above is only part of the sql transaction. thank you in advance.