I have created one flag which is "Generic Hold" on customer card when i will make this ON , while creating Sales Order for that particular Customer after selecting Customer no " you can not create SO because generic hold is ON"

i have tried with this simple code but its not working
tableextension 50006 "Sales order Ext" extends "Sales Header"
{
fields
{
modify("Sell-to Customer No.")
{
trigger OnAfterValidate()
begin
rec.SetFilter("Sell-to Customer No.", co."No.");
rec.Find();
if co."General Hold" = true then begin
Error('You cannot create sales order as General hold is on');
Message('After');
end;
end;
var
co: Record Customer;
so: page "Sales Order";
Kindly Please Suggest
Thank You