using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Conexion"].ToString())){
 string query = @"SELECT idVariableFormula, Nombre, Abreviacion FROM VariableFormula";
 conn.Open();
 SqlCommand sc = new SqlCommand(query, conn);
 SqlDataReader reader = sc.ExecuteReader();
 SqlDataReader reader2 = sc.ExecuteReader();
 ...

There is already an open DataReader associated with this Command, you must close it first.