Not a clue why it is not working but ebay obviously knows about it.

Probably one of the many glitches that abound at present.

Reviews are for products, not sellers

Do you mean you are trying to leave feedback for a seller or a review for an item ?

(Given the message you get, I assume you want to review the product, not write feedback for the seller)

Just in case you were wanting to leave feedback though, which a different thing done in a different place