Guru do Excel e Power BI

Sub InserirFim()

Dim wb As Workbook: Set wb = ThisWorkbook
Dim ws As Worksheet: Set ws = wb.ActiveSheet

Dim ultimaLinha As Long
ultimaLinha = ws.Cells(ws.Rows.Count, "B").End(xlUp).Row + 1

Dim i As Long

For i = 1 To ultimaLinha

If IsEmpty(ws.Cells(i, 2)) Then 'Se Vazio
    If Not IsEmpty(ws.Cells(i - 1, 2)) Then
        If ws.Cells(i - 1, 2) <> "FIM" Then
            ws.Cells(i, 2) = "FIM"
        End If
    End If
End If

Next i

End Sub