C# error in query linq

When I dummy up your code in VS 2019:

private void FrmMain_Shown(object sender, EventArgs epp)
    {
    var t = (from e in context.T_Att
             group e by new { e.Cod_R, e.Cod_C, e.O_Tot } into g
             select new
                 {
                 Ris = g.Key.Cod_R,
                 Cli = g.Key.Cod_C,
                 O_N = g.Sum(pc => pc.O_Tot) * 1236,
                 O_S = g.Sum(pc => pc.O_Tot).ToString(),
                 Tot_rec = g.Count()
                 }).ToList();
    }
public class Context
    {
    public List<foo> T_Att { get; set; } = new List<foo>();
    }
public class foo
    {
    public int Cod_R { get; set; }
    public int Cod_C { get; set; }
    public int O_Tot{ get; set; }
    }
public Context context = new Context();

I get no errors: The chances are that the error is referring to code above the fragment you show, and on;y showing up as the first line of that code because it can't work out what you meant to write.

So start by looking at the lines immediately above this: is there a missing "}" or similar?