mandatory cells

Try this then.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

    Dim i As Long, myarr, x
    
    If Target.Row < 3 Then Exit Sub
    
    If Target.Column <> 3 And Target.Column <> 4 Then
    
        myarr = Array(" una data ", " il reparto ", " l'operatore ", " il tempo ")
        
        x = Split("1,2,5,6", ",")
        
        For i = 0 To 3
            If Cells(Target.Row, CInt(x(i))) = vbNullString Then
                Application.EnableEvents = False
                MsgBox "Devi inserire" & myarr(i) & "nella cella " & Chr(IIf(i < 2, i, i + 2) + 65) & Target.Row & " ", vbCritical, "Errore!"
                Application.Goto Cells(Target.Row, IIf(i < 2, i + 1, i + 3))
                Application.EnableEvents = True
                Exit For
            End If
        Next
    End If
End Sub