Why this LINQ is not working in production ?

var selected = from s in allStudents.AsEnumerable()
                               join c in allPaymentStudentId.AsEnumerable()
                               on s.Field<string>("StudentId") equals c.Field<string>("StudentId")
                               select s;
                bool existsRecord = false;
                foreach(var sel in selected)
                {
                    existsRecord = true;
                
                }
                if (existsRecord)
                {
                    DataTable result = selected.CopyToDataTable();
                    allStudents = result;
                }

            }

            Session["AllStudents"] = allStudents;
            
            radStudent.DataSource = null;
            radStudent.Rebind();