As you likely have already found out, Amazon doesn't allow people to leave a review on an ebook before it's officially available for sale. Unfortunately, Booksprout cannot tell the difference between a book that's on pre-order and one that's live, so you may occasionally run into this issue if the author requests your review too early.
If you find yourself in this situation, there's a couple things you can try.
1. Leave a review on the paperback (if available)
Some authors make their paperbacks available early. If possible, switch to the paperback on Amazon by clicking the "paperback" buying option.
Once there, you should be able to leave a review on Amazon as usual.
If there's no paperback available, the only other option is to simply wait for the book to be on sale as an ebook before you can leave your review. You can see when it's available for sale by looking at the right hand side of the book's Amazon page.