It will be because the item is not live on the website yet. If you go to your account and find it listed under the items to be reviewed but click on write review instead of the item link it should take you the page to write it.
If it's a tester you can scroll to the bottom of your account page and there's often a bit of blurb in the Community section that says you've been sent an item and gives you a link to review it. If it's super secret it might not show up there either.