How can I stop my vba code from giving me an error 424?

I've written a code that does many things. Essentially it loops through each worksheets starting with MW, in these sheets, it deletes some columns, does some operations and changes some column names. Right now, The code works, but after adding the loop through worksheets, I get an error 424 on the "If not Rng Is Nothing Then Rng.EntireColumn.Delete"

How can I fix this?

My theory is that my ws loop doesn't work well, so the code can't work since the sheet is already processed

Here's my code

    Dim Cl As Range, Rng As Range
Dim Cl2 As Range, Rng2 As Range
Dim Cl3 As Range, Rng3 As Range
Dim c As Range
Dim Cl4 As Range, Rng4 As Range
Dim Lastrow As Long
Dim ws As Worksheet
For Each ws In ActiveWorkbook.Worksheets
If ws.Name Like "MW*" Then
For Each Cl In Range("A1:J1")
Select Case Cl.Value
Case "#", "Coupler Detached", "Coupler Attached", "Host Connected", "End Of File", "ms"
If Rng Is Nothing Then Set Rng = Cl Else Set Rng = Union(Rng, Cl)
End Select
Next Cl
If Not Rng Is Nothing Then Rng.EntireColumn.Delete
For Each Cl4 In Range("D1")
Select Case Cl4.Value
Case "Abs Pres (kPa) c:1 2"
If Rng4 Is Nothing Then Set Rng4 = Cl4 Else Set Rng4 = Union(Rng4, Cl4)
End Select
Next Cl4
If Not Rng4 Is Nothing Then
Application.ScreenUpdating = False
Lastrow = Cells(Rows.Count, "D").End(xlUp).Row
For Each c In Range("D2:D" & Lastrow)
c.Value = c.Value * 0.101972
Next
Application.ScreenUpdating = True
End If
For Each Cl2 In Range("A1:J1")
Select Case Cl2.Value
Case "Abs Pres (kPa) c:1 2"
If Rng2 Is Nothing Then Set Rng2 = Cl2 Else Set Rng = Union(Rng, Cl2)
End Select
Next Cl2
If Not Rng2 Is Nothing Then Rng2.Value = ("LEVEL")
For Each Cl3 In Range("A1:J1")
Select Case Cl3.Value
Case "Temp (°C) c:2"
If Rng3 Is Nothing Then Set Rng3 = Cl3 Else Set Rng = Union(Rng, Cl3)
End Select
Next Cl3
If Not Rng3 Is Nothing Then Rng3.Value = ("TEMPERATURE")
End If
Next ws

Object Variables in Loops

• The main issue was that you cannot combine cells with an invalid range so you need to 'reset' the rng* variables i.e. explicitly set them to nothing. For example, in the first iteration, there were cells combined into rng. Now you delete rng and the variable's state is still Not Nothing although you have deleted the range making it invalid. In the next iteration, you try to combine this invalid range (which is Not Nothing) with a matching cell via Union so the error occurs. BTW, even if you didn't delete the range, again, an error would occur because you cannot combine ranges from different worksheets.
• In the following code, pay attention to how this (Set rng = Nothing) is done after each stage per worksheet when only a single rng variable is used.
• Also, note how ws and cell are 'safe', they don't need to be reset, since For Each... could be translated as something like Set ws = WhatEverWorksheet or Set cell = WhatEverCell.

<!-- language: lang-vb -->

Sub ALot()
Dim wb As Workbook: Set wb = ThisWorkbook ' workbook containing this code
' If it's not, change back to 'ActiveWorkbook'.
Application.ScreenUpdating = False
Dim ws As Worksheet, rng As Range, cell As Range
For Each ws In wb.Worksheets
If ws.Name Like "MW*" Then
' Delete columns.
For Each cell In ws.Range("A1:J1").Cells
Select Case CStr(cell.Value)
Case "#", "Coupler Detached", "Coupler Attached", _
"Host Connected", "End Of File", "ms"
If rng Is Nothing Then Set rng = cell Else Set rng = Union(rng, cell)
End Select
Next cell
If Not rng Is Nothing Then
rng.EntireColumn.Delete
Set rng = Nothing ' reset
End If
' Check 4th column.
Set cell = ws.Range("D1")
If CStr(cell.Value) = "Abs Pres (kPa) c:1 2" Then
Set rng = ws.Range("D2", ws.Cells(ws.Rows.Count, "D").End(xlUp))
rng.Value = ws.Evaluate(rng.Address & "*0.101972")
Set rng = Nothing ' reset
End If
' Check "Abs Pres (kPa) c:1 2".
For Each cell In ws.Range("A1:J1").Cells
Select Case CStr(cell.Value)
Case "Abs Pres (kPa) c:1 2"
If rng Is Nothing Then Set rng = cell Else Set rng = Union(rng, cell)
End Select
Next cell
If Not rng Is Nothing Then
rng.Value = "LEVEL"
Set rng = Nothing ' reset
End If
' Check "Temp (°C) c:2".
For Each cell In ws.Range("A1:J1").Cells
Select Case cell.Value
Case "Temp (°C) c:2"
If rng Is Nothing Then Set rng = cell Else Set rng = Union(rng, cell)
End Select
Next cell
If Not rng Is Nothing Then
rng.Value = "TEMPERATURE"
Set rng = Nothing ' reset
End If
'Else ' is not like "MW*"; do nothing
End If
Next ws
Application.ScreenUpdating = True
MsgBox "Worksheets processed.", vbInformation
End Sub