if you remove the paymentTermsId do you get the same error? I have virtually the same body working correctly and I know for certain the GUID for the paymentTermsId is present in the company I am POSTing to. I use variables in my example body:
{
"number": "1009",
"customerNumber": "10000",
"currencyCode": "USD",
"orderDate": "2021-12-31",
"paymentTermsId": "{{paymentTermsId}}",
"salesOrderLines": [
{
"itemId": "{{itemId}}",
"quantity": 1
}
]
}