Autofill method of range class failed - help needed for my code - vba

Question

I've been trying to make this work. It worked fine on my x64 version of office but not the x86 on my colleagues' computers. Can anyone gimme a fix please?
The VBA engine highlighted Range("AV5").AutoFill Destination:=Range("AV5:AV" & NoOfClients) as the cause
Private Sub Check_Cases_Click()
Dim NoOfClients As Long
Application.DisplayAlerts = False
CO_Select = Application.InputBox("Please input the name of caseworker you would like to check on.", "Caseworker Name")
Range("A2").value = CO_Select
Application.ScreenUpdating = False
NoOfClients = Range("C2").value
CO_Name = Range("A2").value
CheckCaseMsg = MsgBox(CO_Name & ", there are " & NoOfClients & " clients under your name." & vbNewLine & vbNewLine & _
"System will now calculate all your active cases and display " & vbNewLine & _
"all the clients for your information." & vbNewLine & vbNewLine & _
"Confirm?", vbYesNo, "Case Checking")
If CheckCaseMsg = vbNo Then
Exit Sub
End If
If CheckCaseMsg = vbYes Then
'Remove the filters if one exists
'=========================================
If ActiveSheet.FilterMode Then
Selection.AutoFilter
End If
Clear
Startup_Formula
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Range("AV5").AutoFill Destination:=Range("AV5:AV" & NoOfClients)
Application.Calculation = xlCalculationAutomatic
Range("GI_Table[[#All],[Client number]]").Copy
Range("GI_Table[[#All],[Client number]]").PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
Application.ScreenUpdating = True
ActiveSheet.ListObjects("GI_Table").Range.AutoFilter Field:=2, Criteria1:= _
Array("ACTIVE", "INACTIVE", "RENEWED"), Operator:=xlFilterValues
GI_CustomSort
GI_CustomSort
MsgBox "Case Checking Ready", vbInformation, "Ready"
End If
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
Application.DisplayAlerts = True
End Sub

Answer 1

Instead of just Range(".."). you should try
Dim sht as Worksheet
Set sht = Worksheets("Name")
sht.Range("..")
and so on.

Answer 2

You will get the Autofill method of range class failed error if the value of NoOfClients is 5. The end row is the same as the start row. Here is a simple way to reproduce the error.
Sub Sample()
NoOfClients = 5
Range("AV5").AutoFill Destination:=Range("AV5:AV" & NoOfClients)
End Sub
An alternative to .Autofill is to directly write to the range. For ex:
Range("AV5:AV" & NoOfClients).Formula = Range("AV5").Formula

Answer 3

Ok I know what's the problem now, I think.
I limited my scroll area in the Sub for Worksheet_Activate and the value of NoOfClients exceeded the allowed amount!
So I deleted that and I think it's ok now! This is my final code (with the scroll area becoming dynamic now)
Private Sub Check_Cases_Click()
Dim NoOfClients As Long
ActiveSheet.ScrollArea = ""
Application.Calculation = xlCalculationAutomatic
Application.DisplayAlerts = False
CO_Select = Application.InputBox("Please input the name of caseworker you would like to check on.", "Caseworker Name")
Range("A2").value = CO_Select
Application.ScreenUpdating = False
NoOfClients = Range("C2").value
CO_Name = Range("A2").value
CheckCaseMsg = MsgBox(CO_Name & ", there are " & NoOfClients & " clients under your name." & vbNewLine & vbNewLine & _
"System will now calculate all your active cases and display " & vbNewLine & _
"all the clients for your information." & vbNewLine & vbNewLine & _
"Confirm?", vbYesNo, "Case Checking")
If CheckCaseMsg = vbNo Then
Exit Sub
End If
If CheckCaseMsg = vbYes Then
'Remove the filters if one exists
'=========================================
If ActiveSheet.FilterMode Then
Selection.AutoFilter
End If
Clear
Startup_Formula
'Fill down the formula for n times where n= No of Clients of the Caseworker'
'=============================================================================
Dim Sht As Worksheet
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual
Set Sht = Worksheets("Grand Info Sheet")
NoOfClients = Range("C2").value
NoOfClientsAdjusted = NoOfClients + 4
Sht.Range("AV5").AutoFill Destination:=Sht.Range("AV5:AV" & NoOfClientsAdjusted)
Application.Calculation = xlCalculationAutomatic
Range("GI_Table[[#All],[Client number]]").Copy
Range("GI_Table[[#All],[Client number]]").PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
Application.ScreenUpdating = True
GI_CustomSort
MsgBox "Case Checking Ready", vbInformation, "Ready"
Range("A1").Select
End If
ActiveSheet.ScrollArea = "A1:AW" & NoOfClientsAdjusted + 5
Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
Application.DisplayAlerts = True
End Sub