Got stuck at creating a code for Ifna, sumif and sumproduct

Hi All, would like to create a code for Ifna, sumif and sumproduct. 

Tried to create a code replicate the excel formula below:

IFNA(IF(LEN(B1)>1,

SUMIF(Sheet1!E:I,B2,Sheet1!I:I),

SUMPRODUCT(MAX(--('[Credit Limit Requests (no existing SAP IDs).xlsx]No SAP ID'!$B2:$B=B2)*--('[Credit Limit Requests (no existing SAP IDs).xlsx]No SAP ID'!$S2:$S=SUMPRODUCT(MAX(--('[Credit Limit Requests (no existing SAP IDs).xlsx]No SAP ID'!$B2:$B1000=B2)*'[Credit Limit Requests (no existing SAP IDs).xlsx]No SAP ID'!$S2:$S)))*'[Credit Limit Requests (no existing SAP IDs).xlsx]No SAP ID'!$M2:$M))),

"Credit limit not provided to this customer")

Got stuck whilst trying to create the code:

1st hurdle: To insert the IF into IFNA

ws.Range("C44") = WorksheetFunction.IfNa( _
If Len(ws.Range("C10")) > 1 Then
WorksheetFunction.SumIf(exA, ws.Range("C15"), exB)
Else

2nd hurdle: No clue how to insert a MsgBox if value equals #N/A

MsgBox "Credit limit not provided to this customer"

Dim ws As Worksheet 'current sheet
Dim exportWb As Workbook
Dim clRequestWb As Workbook
Dim exportWs As Worksheet
Dim clRequestWs As Worksheet
Dim exportWslastRow As Long

Dim exA As Range
Dim exB As Range
Dim cLB As Range
Dim cLS As Range
Dim cLM As Range


Set ws = Sheet1
Set exportWb = Workbooks.Open(Environ("Userprofile") & "\Desktop\export.xlsx")
Set exportWs = exportWb.Sheets("Sheet1")
exportWslastRow = exportWs.Cells(exportWs.Rows.Count, "B").End(xlUp).Row

Set exA = exportWs.Range("E2:I" & exportWslastRow)
Set exB = exportWs.Range("I2:I" & exportWslastRow)

Set clRequestWb = Workbooks.Open("V:\Finance\Systems-Risk-ERM\OrderToCash\C2C\Corp Credit Control\1. Credit Limit Requests\Credit Limit Requests (no existing SAP IDs).xlsx")
Set clRequestWb = ActiveWorkbook
Set clRequestWs = clRequestWb.Sheets("No SAP ID")
ThisWorkbook.Activate

Set cLB = clRequestWs.Cells(clRequestWs.Rows.Count, "B").End(xlUp).Row
Set cLS = clRequestWs.Cells(clRequestWs.Rows.Count, "S").End(xlUp).Row
Set cLM = clRequestWs.Cells(clRequestWs.Rows.Count, "M").End(xlUp).Row

ws.Range("C44") = WorksheetFunction.IfNa( _

If Len(ws.Range("C10")) > 1 Then

WorksheetFunction.SumIf(exA, ws.Range("C15"), exB)

Else

Application.WorksheetFunction.SumProduct (WorksheetFunction.Max(--(cLB = clRequestWs.Range("B2")) *
--(cLS = Application.WorksheetFunction.SumProduct(WorksheetFunction.Max(--(cLB = clRequestWs.Range("B2")) * _
cLS))) * cLM))

End If

Thanks in advance and appreciate the help