While not the complete answer, your current code may not evaluate correctly for accountCol. Consider the following update:
Dim Accountcol As String = args.Value
'Reminder - (2) column is 3rd column in excel - need to count "0" as a digit
Dim CostCentercol As String = api.Parser.DelimitedParsedValues(6)
Dim AccountcolValue As Integer
'Attempt to convert Accountcol to an integer
If Integer.TryParse(Accountcol, AccountcolValue) Then
'Identify PL Accounts (greater than 39999)
If AccountcolValue > 39999 Then
'Need to bring in Cost Center
Return CostCentercol
Else
'Use Account as UD2 source
Return Accountcol
End If
Else
Return Accountcol
End If