The issue is with the purchaseorderline table. I am trying to insert values for Item, PurchaseOrderRecordID, Rate, and Quantity.
Before trying to insert to the purchaseorderline table I have succesfully inserted a Purchase Order to the purchaseorder table.
Here are the parameters for the failing API call.
parameters = {
    'strCompanyAccount': companyAccount,
    'strLogin': username,
    'strPassword': password,
    'strSessionID': '',
    'strTable': 'purchaseorderline',
    'arrInsertFieldsArray': arrNames,
    'arrInsertValueArray': arrValues
 }