VBA: Unable to get the Match property of the WorkSheet function class

it helps some times I know that a similar problem was already discussed here: Why is VLookup in VBA failing with runtime error 1004? , This might be a shot in the dark but I believe your error is here

PosIdent = "IdSelect" & "/" & Cnt + 1

PosIdent = Range("IdSelect").Value & "/" & Cnt + 1

For Cnt = 0 To ServCnt + 1
    PosIdent = Range("IdSelect").Value & "/" & Cnt + 1
    ActiveCell.Value = Application.WorksheetFunction.VLookup(PosIdent, Data, 15, False)
    ActiveCell.Offset(1, 0).Activate
Next Cnt

Option Explicit
Public ServCnt As Integer
Sub Main()

Call Count_Line_Items
Call Count_Total_Rows
Call Write_Services

End Sub
Sub Count_Line_Items()

'Counts the number of line items of a consulting project to determine the space needed on the invoice form

 Dim Cell As Range
 Dim PosCnt As Integer
 Dim ExpCnt As Integer

 PosCnt = 0
 ServCnt = 0
 ExpCnt = 0

'Counting all project positions for the chosen project number
For Each Cell In Range("ProjectList")
   If Cell.Value = Range("IdSelect") Then
       PosCnt = PosCnt + 1
    End If
Next Cell

MsgBox "Total number of line items: " & PosCnt

'Counting all positions of that project that are consulting services
For Each Cell In Range("ProjectList")
   If Cell.Value = Range("IdSelect").Value And Cell.Offset(0, 3).Value = "Service" Then
    ServCnt = ServCnt + 1
   End If
Next Cell

MsgBox "Total number of consulting services: " & ServCnt

'Calculating number of expense items
ExpCnt = PosCnt - ServCnt

MsgBox "Total number of expenses: " & ExpCnt
End Sub

Sub Count_Total_Rows()

Dim Current_RowCnt As Integer
Dim Target_RowCnt As Integer
Dim Diff_Rows As Integer

Target_RowCnt = 62

'Counting the rows in the print area and calculating difference to target
Range("Print_Area").Select
Current_RowCnt = Selection.Rows.Count
Diff_Rows = Target_RowCnt - Current_RowCnt
    If Diff_Rows > 0 Then
        MsgBox "We need to add " & Diff_Rows & " rows!"
    ElseIf Diff_Rows < 0 Then
        MsgBox "We need to delete " & -Diff_Rows & " rows!"
    Else
        MsgBox "Nothing needs to be done; all good!"
    End If
End Sub

Sub Write_Services() 'Looks up services on data sheet and writes them to invoice sheet Dim Cnt As Integer Dim PosIdent As String Dim Data As Range

Cnt = 0
'Building position identifier

Sheets("Input").Select
ActiveSheet.Range("D26:AD151").Select
Set Data = Selection
PosIdent = Range("IdSelect").Value & "/" & Cnt + 1

Sheets("Invoice").Select
ActiveSheet.Range("Service_Title").Offset(1, 0).Activate
'There is still an issue with the counter (line number won't increment by 1 if cnt range is incremented by 1
For Cnt = 0 To ServCnt + 1
    ActiveCell.Value = Application.WorksheetFunction.VLookup(PosIdent, Data, 15, False)
    ActiveCell.Offset(1, 0).Activate
    Cnt = Cnt + 1
Next Cnt
End Sub

I think the issue was by ths following , Sorry to answer my own question. Figured it out by accident.
The problem was that I had my textbox (txtFilePath) grouped together with another textbox on my worksheet. When I ungrouped them everything worked fine. Can anyone explain to me why the grouping would make a difference?

seems to work fine Error handling in VBA is a bit tricky. Basically, you are handling an error inside your loop, but you did not exit the procedure. You hit the error once, you handle it and you continue the loop - but you are still in the same sub - so the next error will raise regardless of the Error-handling mechanism that you had put in place..
To continue in your same procedure/loop after you caught and handled an error, you need to invoke in any way the Resume statement. After invoking Resume, your Error-Handling mechanism is again valid. In your case, you should do the following:

    On Error Goto ErrHandler
    Do While Sheets("Sheet1").Range("A" & n) <> ""
        .... ' your loop
NextN:
        n = n + 1
    Loop

    ...

    Exit Sub

ErrHandler:
    Resume NextN 'you need to call Resume, to continue your loop with valid error handling.

End Sub

hope this fix your issue a = Application.Small(m, j) will surely return an Error Code when j is actually bigger that the size of te range m. In your code, the range m = Range("E3:E40") has 38 cells, but j can go as high as 38 * 172.
Then you try to call Match with an error code as the first parameter a. This resuts in run-time error. Note here that Application.Match would result in an error code while WorksheetFunction.Match raises a run-time error.

a = Application.Small(m, k) ' <--- k, not j

To fix the issue you can do For those interested, this code works fine. I was getting an error because I had an #value error in the sum range.