I see couple of problems
You are using xlDown to find the last cell. I would recommend seeing THIS link on how to find last row.
You are getting the error Runtime Error 9 Subscript out of range because Excel cannot find the sheet that you are referring to. Please ensure that the sheet exists. If you can visually see the Untitled sheet then I believe the sheet name has leading or trailing spaces.
Once you solve that issue, the next error that you will get is Runtime error 438: Object doesn't support this property or method. And that is because you are using cell instead of cells. For example .cell(i, 1) should be .Cells(i, 1) and .cell(i + 1, 1) should be .Cells(i + 1, 1)
Declare your objects else if Sheets("Untitled") is not the active sheet then Rows(i).Delete will delete from the wrong sheet ;)
Avoid deleting the rows in a loop. It will only make your code slower. See how I have used delRange in the code below.
Fix these things and you will be good to go
Note:
Option Explicit
Sub em()
Dim delRange As Range
Dim ws As Worksheet
Dim i As Long, LRow As Long
Set ws = ThisWorkbook.Sheets("Untitled")
With ws
LRow = .Range("A" & .Rows.Count).End(xlUp).Row
For i = LRow To 2 Step -1 '~~> Even 2 To LRow will work :)
If .Cells(i, 1) = .Cells(i + 1, 1) Then
If delRange Is Nothing Then
Set delRange = .Rows(i)
Else
Set delRange = Union(delRange, .Rows(i))
End If
End If
Next i
If Not delRange Is Nothing Then delRange.Delete
End With
End Sub