Generally, toys can have up to 3 verified (free assigned) reviews for them, and lingerie items can have 1 verified review. So if there are already that many assigned requests out, no more can be accepted. Just means that particular item is already spoken for.

Here is what the Review Program FAQ says about that:

...I just requested a product to review and the system told me I had three outstanding reviews…but I don’t. What’s up with that?
It’s not you, it’s the product. We can only allow so many free products for review, three is our max. If you see a product without any reviews on it, but still get this notice, it means there could be pending reviews on the product. Also, sorry to say, lingerie/clothing items only get one free review per product. ...


The other thing that sometimes happens - if it hasn't been 30 days since your last requested item, you can't get another free assignment. You can only request another free assignment after 30 days. Or, if you have an outstanding assignment that you haven't completed, you can't request another until you complete the current assignment.

Hopefully that answers your question.