Worksheets.Add in UDF Not Working - vba

Question

I have a UDF that can be called from within a cell in my excel workbook. I need it to add a worksheet at the end of the workbook. I have used sheets.add multiple times in my VBA script, but never in a function called from inside a cell and this is apparently causing some issue.
The function accepts an optional parameter for file path of the workbook in which to add the sheet, and if the user leaves this blank I want to default to the active workbook.
Below is the relevant code... What am I doing wrong?
Public Function onesheet(Optional filepath As String)
Dim wb As Workbook
Dim ws As Worksheet
If filepath = "" Then
Set wb = ActiveWorkbook
Set target_ws = wb.Sheets.Add(after:=wb.Sheets(wb.Sheets.Count))
End If
The function is being called from the cell with
=onesheet()

Answer 1

A function (UDF) has one role: compute a value and return that value to the cell (or formula/expression) that called it.
This is a function:
Public Function Foo(ByVal bar As String) As String
Foo = "Hello, " & bar
End Function
You can use it in a worksheet cell like this:
=Foo("dsdavidson")
And every time Excel recalculates that cell's value, it calls the UDF, making the cell's value read Hello, dsdavidson.
Functions don't have side-effects. Functions don't modify other cells. Functions take input, process it, and output a result.
What you're doing wrong, is using a UDF as if it were a macro.
Change your Function for a Sub, and don't call it from within a cell. Make a button to call it instead. Or whatever rocks your boat. But you can't have a cell formula that adds a worksheet to the workbook every time it recalculates.
Macros need to be Public and parameterless. So you'll want to take your optional parameter value from a specific cell, or display a form that lets the user pick from a list of available opened workbooks - and then call your procedure and pass the user's selection as a parameter.
Quite possibly the macro code could end up looking something like this (YMMV):
Public Sub AddWorksheet()
With New PromptForm
.Show
If .Cancelled Then Exit Sub
OneSheet .SelectedBook
End With
End Sub

Answer 2

You cannot add sheets through user defined function.
Here are the limitations of User Defined Functions.
A user-defined function called by a formula in a worksheet cell cannot change the environment of Microsoft Excel. This means that such a function cannot do any of the following:
1) Insert, delete, or format cells on the spreadsheet.
2) Change another cell's value.
3) Move, rename, delete, or add sheets to a workbook.
4) Change any of the environment options, such as calculation mode or screen views.
5) Add names to a workbook.
6) Set properties or execute most methods.
For more details visit this site...
https://support.microsoft.com/en-in/help/170787/description-of-limitations-of-custom-functions-in-excel