I think you set the Transaction Type in request body incorrectly. Below is an example of a SOAP request to create a Customer Payment record

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"

xmlns:platformCore="urn:core_2021_1.platform.webservices.netsuite.com"

xmlns:platformMsgs="urn:messages_2021_1.platform.webservices.netsuite.com"

xmlns:tranCust="urn:sales_2021_1.transactions.webservices.netsuite.com">

<soapenv:Header>

<platformMsgs:passport>

<platformCore:email>your_email@example.com</platformCore:email>

<platformCore:password>YourPassword</platformCore:password>

<platformCore:account>123456</platformCore:account>

<platformCore:role internalId="3" />

</platformMsgs:passport>

</soapenv:Header>

<soapenv:Body>

<platformMsgs:add>

<platformMsgs:record xsi:type="tranCust:CustomerPayment" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

<tranCust:customer internalId=""/> <!-- Internal ID of the Customer -->

<tranCust:payment>

<tranCust:paymentMethod internalId="1"/> <!-- Internal ID of the payment method -->

<tranCust:amount>500.00</tranCust:amount> <!-- Payment Amount -->

</tranCust:payment>

<tranCust:applyList>

<tranCust:apply>

<tranCust:doc internalId=""/> <!-- Internal ID of the Invoice -->

<tranCust:amount>500.00</tranCust:amount>

</tranCust:apply>

</tranCust:applyList>

</platformMsgs:record>

</platformMsgs:add>

</soapenv:Body>

</soapenv:Envelope>