Bunu silip,
xtOncekiAy = cbHesAylar - "-1"

Bu kodu ekleyip denermisiniz...

Private Sub cbHesAylar_AfterUpdate()

Dim lsRw As Integer, shRw As Integer
Dim lstAy As String
Dim syc As Double
Dim wsh As Worksheet

On Error Resume Next

syc = 0
lstAy = vbNullString
lsRw = Me.cbHesAylar.ListIndex
If lsRw = 0 Then
lstAy = "ARALIK"
Else
lstAy = Me.cbHesAylar.List((lsRw - 1))
Set wsh = ThisWorkbook.Worksheets("BIRLIK-2024")
With wsh
shRw = .Range("A21:A32").Find(What:=lstAy, LookIn:=xlValues, LookAt:=xlWhole, SearchOrder:=xlByRows).Row
syc = CDbl(.Range("M" & shRw).Value2)
End With
End If

Me.txtOncekiAy.Value = lstAy
Me.txtIlkOkuma.Value = syc

End Sub