32 bit and 64 bit compatability

So I am working on a project for my father-in-law's business and I just sent what I have right now to him to test it and he came up with an error saying that it needs to be updated to function with 64bit. I have put over 40 hours into this worksheet and I don't want to lose it all. What kind of changes would I need to make to make it compatible?

Here is the spreadsheet if that helps. (Some of my macros aren't complete yet either so just ignore those ones.) Thanks for any help you can provide.
Inventory Management Accounting.xlsm

Are you actually using the code in the NoTitle module?

I can't see it being called anywhere.

It'd probably be a better idea to keep it and change it to be compatible.

No - that's an example of how to make an API call compatible (its not an API function you are using). You need to apply the same principle to the actual function declarations you are using.

Not quite. Your module would become something like this:

#If Win64 Then
    Private Declare PtrSafe Function FindWindow Lib "user32" Alias "FindWindowA" _
      (ByVal lpClassName As String, ByVal lpWindowName As String) As LongPtr
    
    Declare PtrSafe Function SetWindowLong Lib "user32" Alias "SetWindowLongPtrA" _
        (ByVal hwnd As LongPtr, ByVal nIndex As Long, ByVal dwNewLong As LongPtr) As LongPtr
    
    Private Declare PtrSafe Function GetWindowLong Lib "user32" Alias "GetWindowLongPtrA" _
                    (ByVal hwnd As LongPtr, ByVal nIndex As Long) As LongPtr
                    
    Private Declare PtrSafe Function SetWindowPos Lib "user32" _
          (ByVal hwnd As LongPtr, ByVal hWndInsertAfter As LongPtr, _
          ByVal x As Long, ByVal y As Long, ByVal cx As Long, _
          ByVal cy As Long, ByVal wFlags As Long) As Long
      
    Private Declare PtrSafe Function GetWindowRect Lib "user32" _
      (ByVal hwnd As LongPtr, lpRect As RECT) As Long
      
#Else
    Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
      (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
    
    Private Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
      (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
    
    Private Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
      (ByVal hwnd As Long, ByVal nIndex As Long) As Long
    Private Declare Function SetWindowPos Lib "user32" _
      (ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
      ByVal x As Long, ByVal y As Long, ByVal cx As Long, _
      ByVal cy As Long, ByVal wFlags As Long) As Long
    Private Declare Function GetWindowRect Lib "user32" _
      (ByVal hwnd As Long, lpRect As RECT) As Long
#End If

Private Const GWL_STYLE = (-16)
Private Const WS_CAPTION = &HC00000
Private Const WS_MAXIMIZEBOX = &H10000
Private Const WS_MINIMIZEBOX = &H20000
Private Const WS_SYSMENU = &H80000


Private Enum ESetWindowPosStyles
  SWP_SHOWWINDOW = &H40
  SWP_HIDEWINDOW = &H80
  SWP_FRAMECHANGED = &H20
  SWP_NOACTIVATE = &H10
  SWP_NOCOPYBITS = &H100
  SWP_NOMOVE = &H2
  SWP_NOOWNERZORDER = &H200
  SWP_NOREDRAW = &H8
  SWP_NOREPOSITION = SWP_NOOWNERZORDER
  SWP_NOSIZE = &H1
  SWP_NOZORDER = &H4
  SWP_DRAWFRAME = SWP_FRAMECHANGED
  HWND_NOTOPMOST = -2
End Enum


Private Type RECT
  Left As Long
  Top As Long
  Right As Long
  Bottom As Long
End Type

Sub Title_Show()
  Application.DisplayScrollBars = True
  ActiveWindow.DisplayHeadings = True
  ShowTitleBar True
End Sub

Sub Title_Hide()
  Application.DisplayScrollBars = False
  ActiveWindow.DisplayHeadings = False
  ShowTitleBar False
End Sub

Sub ShowTitleBar(bShow As Boolean)
    #If Win64 Then
        Dim xlHnd As LongPtr
        Dim lStyle As LongPtr
    #Else
      Dim xlHnd As Long
      Dim lStyle As Long
    #End If
  Dim tRect As RECT
  
  xlHnd = Application.hwnd

  '// Get the window's position:
  GetWindowRect xlHnd, tRect

  '// Show the Title bar ?
  If Not bShow Then
    lStyle = GetWindowLong(xlHnd, GWL_STYLE)
    lStyle = lStyle And Not WS_SYSMENU
    lStyle = lStyle And Not WS_MAXIMIZEBOX
    lStyle = lStyle And Not WS_MINIMIZEBOX
    lStyle = lStyle And Not WS_CAPTION
  Else
    lStyle = GetWindowLong(xlHnd, GWL_STYLE)
    lStyle = lStyle Or WS_SYSMENU
    lStyle = lStyle Or WS_MAXIMIZEBOX
    lStyle = lStyle Or WS_MINIMIZEBOX
    lStyle = lStyle Or WS_CAPTION
  End If

  SetWindowLong xlHnd, GWL_STYLE, lStyle

  Application.DisplayFullScreen = Not bShow

  '// Ensure the style is set and makes the xlwindow the
  '// same size, regardless of the title bar.
  SetWindowPos xlHnd, 0, tRect.Left, tRect.Top, tRect.Right - tRect.Left, _
    tRect.Bottom - tRect.Top, SWP_NOREPOSITION Or SWP_NOZORDER Or SWP_FRAMECHANGED
End Sub