VBA loop to take value if field is blank - excel

Question

Need some solution in VBA ->If the blank value in column A then takes value from column B.
I wrote some code, but I don't have any idea why this is not working.
dim LastR as Long
LastR = Worksheets("Sheet1").Range("BU" & Worksheets("Sheet1").Rows.Count).End(xlUp).Row
dim i as long
For i = LastR To 2 Step -1
If IsEmpty(Cells(i, "a")) Then Cells(i, "a").Value = Cells(i, "b").Value
Next i

Answer 1

You should check if the value is empty.
See two examples:
Dim LastR As Long
LastR = Worksheets("Sheet1").Range("BU" & Worksheets("Sheet1").Rows.Count).End(xlUp).Row
Dim i As Long
For i = LastR To 2 Step -1
'If Sheets("Sheet1").Cells(i, "a") = "" Then Sheets("Sheet1").Cells(i, 1).Value = Cells(i, 2).Value
If IsEmpty(Sheets("Sheet1").Cells(i, "a").Value) = True Then Sheets("Sheet1").Cells(i, 1).Value = Cells(i, 2).Value
Next i

Answer 2

Loop Through the Cells of a Column
All three versions do the same and are about equally efficient.
Option Explicit
Sub FillEmptiesConstants()
Const wsName As String = "Sheet1" ' Worksheet Name
Const fRow As Long = 2 ' First Row
Const lrCol As String = "BU" ' Last Row Column
Const lCol As String = "A" ' Lookup Column
Const dCol As String = "A" ' Destination Column
Const sCol As String = "B" ' Source Column
Dim wb As Workbook: Set wb = ThisWorkbook ' workbook containing this code
Dim ws As Worksheet: Set ws = wb.Worksheets(wsName)
Dim lRow As Long: lRow = ws.Cells(ws.Rows.Count, lrCol).End(xlUp).Row
If lRow < fRow Then Exit Sub
Dim r As Long
For r = fRow To lRow
If IsEmpty(ws.Cells(r, lCol)) Then
ws.Cells(r, dCol).Value = ws.Cells(r, sCol).Value
End If
Next r
End Sub
Sub FillEmptiesSimple()
Dim wb As Workbook: Set wb = ThisWorkbook ' workbook containing this code
Dim ws As Worksheet: Set ws = wb.Worksheets("Sheet1")
Dim lRow As Long: lRow = ws.Cells(ws.Rows.Count, "BU").End(xlUp).Row
If lRow < 2 Then Exit Sub
Dim r As Long
For r = 2 To lRow
If IsEmpty(ws.Cells(r, "A")) Then
ws.Cells(r, "A").Value = ws.Cells(r, "B").Value
End If
Next r
End Sub
Sub FillEmptiesSimpleWith()
With ThisWorkbook.Worksheets("Sheet1")
Dim lRow As Long: lRow = .Cells(.Rows.Count, "BU").End(xlUp).Row
If lRow < 2 Then Exit Sub
Dim r As Long
For r = 2 To lRow
If IsEmpty(.Cells(r, "A")) Then
.Cells(r, "A").Value = .Cells(r, "B").Value
End If
Next r
End With
End Sub