I am trying to update the vendor table with the following vbscript but I keep getting the error: "530 - A field that is not supported was included in the request. The remainder of the request was processed normally, but the unsupported field was ignored."

Const adOpenStatic = 3
Const adLockOptimistic = 3
Const adUseClient = 3

Dim qConnection
Dim qRecordset
Dim sMsg
Dim sSql

Set qConnection = CreateObject("ADODB.Connection")
Set qRecordset = CreateObject("ADODB.Recordset")

qConnection.Open "DSN=Quickbooks Data;OLE DB Services=-2"
qRecordset.CursorLocation = adUseClient

  sSql = "SELECT * FROM Vendor WHERE Vendor.Name='April Honnies'"

  qRecordset.Open sSql, qConnection, adOpenStatic, adLockOptimistic

  If (qRecordset.EOF) Then
    sMsg = "April was NOT found" & Chr(10)
  Else
    sMsg = "April was found" & Chr(10)
    qRecordset.Fields("CompanyName").value = "Test User Company"
    qRecordset.Update()
  End If

  MsgBox sMsg
 
qRecordset.Close
qConnection.Close

Can't see off hand what the problem is, try looking at the QODBC Message log in the QODBC Setup Screen for the SQL Statement that failed, it will show what is actually being processed. 

Here's one of my examples that does work:

Const adOpenStatic = 3
Const adLockOptimistic = 3
Const adUseClient = 3
 
Dim oConnection
Dim oRecordset
Dim sMsg
Dim Typ
 
Set oConnection = CreateObject("ADODB.Connection")
Set oRecordset = CreateObject("ADODB.Recordset")
 

oConnection.Open "DSN=Quickbooks Data;"
 
oRecordset.CursorLocation = adUseClient
oRecordset.Open "SELECT * FROM InvoiceLine where RefNumber='49'", oConnection, adOpenStatic, adLockOptimistic
sMsg = "************************" & vbCrLf
sMsg = sMsg & "QODBC Driver Test Result:" & vbCrLf
sMsg = sMsg & "Update InvoiceLine Test Script:" & vbCrLf
sMsg = sMsg & "    " & oRecordSet.Fields("CustomerRefFullName") & vbCrLf
sMsg = sMsg & "RefNumber : " & oRecordSet.Fields("RefNumber") & vbCrLf
 
Do While (not oRecordset.EOF)
        if oRecordSet.Fields("InvoiceLineSeqNo") ="2" then
              oRecordset.Fields("CustomFieldInvoiceLineOther1").Value = "999"
              oRecordset.Update
        else
        end if
              sMsg = sMsg & "InvoiceLineSeqNo : " & oRecordSet.Fields("InvoiceLineSeqNo") & vbCrLf
              sMsg = sMsg & "CustomFieldInvoiceLineOther1 : " & oRecordSet.Fields("CustomFieldInvoiceLineOther1") & vbCrLf
 oRecordset.MoveNext
Loop
sMsg = sMsg & "************************" & vbCrLf
MsgBox sMsg
oRecordset.Close
oConnection.Close