They won't ship it to you.
On canada.forever21.com, orders can be placed from outside of Canada, however shipping address must be within Canada. We do not ship orders internationally. If you would like to ship your order outside of Canada, please visit canada.forever21.com.
So basically, the reason you probably can't find it on the US website is because the Canadian items are older stock. The shoes I wanted to buy last week on the US site only were listed today on the Canadian site.
If you REALLY want that necklace, your best bet is to find someone you know in Canada, get them to order it, and mail it over to you.