If cell equals "CR" then make cell to left negative and delete "CR".......

I am sorry my friend but your code is no good.

1. If you are looking at a large range the last thing you want to do is to use a loop.


2.

LR = Cells(rows.count,1).End(xlUp).Row

Will return the last row in column A.

LR = Cells(Rows.Count, "Y").End(xlUp).Row

Will return the last row in column Y.

What makes you sure that Column Y is your longest column?

Perhaps you should use

LCR = Selection.SpecialCells(xlCellTypeLastCell).Row

I will rewrite your code for you.

This is more complicated than your code but hundreds of times faster.
Mainly because it uses the find function rather than a loop.

Sub Negative_Convert()

Application.ScreenUpdating = False

    Cells.Select
    Set RngLook = Selection
    Range("A1").Select
    
    On Error Resume Next
    
    With RngLook
        Set rngFind = .Find("CR", .Cells(1, 1), LookIn:=xlValues, lookat:=xlWhole)
        If Not rngFind Is Nothing Then
        
            rngFind.Offset(0, -1).Value = -rngFind.Offset(0, -1).Value
            rngFind.Value = ""
            
            Do
            Set rngFind = .FindNext(rngFind)
            rngFind.Offset(0, -1).Value = -rngFind.Offset(0, -1).Value
            rngFind.Value = ""
            Loop While Not rngFind Is Nothing
        End If
    End With

    On Error GoTo 0
    
Application.ScreenUpdating = True

End Sub

You said the cell to the left, but Offset(0,1) is the cell to the right. Offset(0,-1) would be the cell to the left. Not sure which you want, adjust the code below accordingly.

Sub pas()
With Range("A:BA")
Set c = .Find("CR", lookat:=xlWhole)
If Not c Is Nothing Then
FirstAdd = c.Address
Do
c.Offset(0, 1).Value = c.Offset(0, 1).Value * -1
c.ClearContents
Set c = .FindNext(c)
Loop While Not c Is Nothing And c.Address <> FirstAdd
End If
End With
End Sub

walruseggmans code would work if you inserted the two lines

On Error Resume Next

and

On Error Goto 0

The reason being that you are Deleting CR, eventually there will be no CR to find giving you an error.

you should also remove all reference to c.address for the same reason.
Turning off screen updates would make it even faster.

Sub pas()

On Error Resume Next
Set c = Range("A:BA").Find("CR", lookat:=xlWhole)
If Not c Is Nothing Then
Do
c.Offset(0, -1).Value = -c.Offset(0, -1).Value
c.ClearContents
Set c = Range("A:BA").FindNext(c)
Loop While Not c Is Nothing
End If

On Error GoTo 0
End Sub

@mehmetcik: Good point, wasn't thinking about that.

@pas: You'll need to also remove And c.Address <> FirstAdd from mehmetcik's modified code.

I cannot see any reason for that.

I pasted the last code into my Peersonal workbook, also a hidden book and ran the macro with no issues.