Answer

Putting the function in the “ThisWorkbook” area can cause the #NAME? problem. Create a new Module (Right Click on the VBAProject Folder, Insert, New Module)
and put the function there instead.

Steps

1. Open the VBA Editor (Alt + F11 on Windows / Fn + Option + F11 on a Mac)
2. Right-click VBAProject
3. Select Insert >> Module

4. Create a Public function inside Module1, for example:

   Public Function findArea(ByVal width as Double, _
                            ByVal height as Double) As Double
       ' Return the area
       findArea = width * height
   End Function
   

5. Call it from a cell, like any other function: =findArea(B12,C12)