Excel VBA - UDF returns 0 or empty or #value - excel

Question

I am creating my customized function. I wrote the code and tested it as “sub and it worked well. Then i converted it into a function to be able to use it in general. Things that i changed are; adding a function declaration, taking input from excel cell and specifying function output. All others remained same.
My function only has one input which is a selected cell from an excel sheet. And i expect that function returns one output. However, it returns 0.
• function declaration. "Function IbpBomLevel(ByVal Target As Range) As Variant
• input of function as selected cell. "ProductID = Target
• output of function. "IbpBomLevel = fullText
I used option explicit to avoid non-exist functionalities. Also, I am sure about the input, function really takes the selected cell as input. But the problem is that in each loop “ProductID must be changed. Hovewer, when i declared that "IbpBomLevel (output of the function) = ProductID and saw that ProductID is the first parameter that user selected from a cell. It means loop not works. When i test it as “sub, i got the result that i want. I am not sure what the problem is.
Option Explicit
Function IbpBomLevel(ByVal Target As Range) As Variant
Dim Wb As Workbook
Dim Ws As Worksheet
Dim MyRange As Range
Dim SourceID As Variant
Dim SourceID2 As Variant
Dim SourceID3 As Variant
Dim Product As Variant
Dim Item As Variant
Dim Location As Variant
Dim Resource As Variant
Dim I As Variant
Dim T As Variant
Dim Z As Variant
Dim X As Variant
Dim Y As Variant
Dim Index As Variant
Dim Index2 As Variant
Dim Index3 As Variant
Dim BomLevel As Variant
Dim FoundCell As Variant
Dim fullText As Variant
Dim ProductID As Variant
ProductID = Target
Set Wb = Workbooks("Kitap.xlsx")
Windows("Kitap.xlsx").Activate
On Error GoTo T_Error
Set Ws = Wb.Worksheets("Production Source Header")
Sheets("Production Source Header").Select
Set MyRange = Worksheets("Production Source Header").Range("B:C")
SourceID = CVar(Application.WorksheetFunction.VLookup(ProductID, MyRange, 2, False))
I = 1
T = 0
Z = 1
If IsEmpty(SourceID) = False Then
Do While (IsEmpty(SourceID) = False) And (T = 0)
BomLevel = Z
Windows("Kitap.xlsx").Activate
Set Ws = Wb.Worksheets("Production Source Header")
Sheets("Production Source Header").Select
Set MyRange = Worksheets("Production Source Header").Range("B:C")
SourceID = CVar(Application.WorksheetFunction.VLookup(ProductID, MyRange, 2, False))
Set FoundCell = ActiveSheet.Range("C:C").Find(What:=SourceID)
If Not FoundCell Is Nothing Then
Index = FoundCell.Row
Location = Cells(Index, 1)
Product = Cells(Index, 2)
Else
End If
X = I
I = I + 1
Windows("Kitap.xlsx").Activate
Set Ws = Wb.Worksheets("Production Source Item")
Sheets("Production Source Item").Select
Set MyRange = Worksheets("Production Source Item").Range("B:B")
SourceID2 = CVar(Application.WorksheetFunction.VLookup(SourceID, MyRange, 1, False))
Do While (IsEmpty(SourceID2) = False) And (I - X = 1)
Set MyRange = Worksheets("Production Source Item").Range("B:B")
SourceID2 = CVar(Application.WorksheetFunction.VLookup(SourceID, MyRange, 1, False))
Set FoundCell = ActiveSheet.Range("B:B").Find(What:=SourceID2)
If Not FoundCell Is Nothing Then
Index2 = FoundCell.Row
Item = Cells(Index2, 1)
Windows("Kitap.xlsx").Activate
Set Ws = Wb.Worksheets("Production Source Header")
Sheets("Production Source Header").Select
Else
End If
Y = I
I = I + 1
Windows("Kitap.xlsx").Activate
Set Ws = Wb.Worksheets("Production Source Resource")
Sheets("Production Source Resource").Select
Set MyRange = Worksheets("Production Source Resource").Range("B:B")
SourceID3 = CVar(Application.WorksheetFunction.VLookup(SourceID, MyRange, 1, False))
Do While IsEmpty(SourceID3) = False And (I - Y = 1)
Set MyRange = Range("B:B")
SourceID3 = CVar(Application.WorksheetFunction.VLookup(SourceID, MyRange, 1, False))
Set FoundCell = ActiveSheet.Range("B:B").Find(What:=SourceID3)
If Not FoundCell Is Nothing Then
Index3 = FoundCell.Row
Resource = Cells(Index3, 1)
Windows("Kitap.xlsx").Activate
Set Ws = Wb.Worksheets("Production Source Header")
Sheets("Production Source Header").Select
Else
End If
I = I + 1
Loop
Loop
fullText = fullText & " Location: " & Location & " // Header: " & Product & " // Item: " & Item & " // Resource: " & Resource
Z = Z + 1
ProductID = Item
Set MyRange = Worksheets("Production Source Header").Range("B:C")
SourceID = (Application.WorksheetFunction.VLookup(ProductID, MyRange, 2, False))
T_Error:
If Err.Number = 1004 Then
On Error Resume Next
T = 1
Else
End If
Loop
IbpBomLevel = fullText
Else
MsgBox ("Bom Missing")
End If
End Function