Did you buy from a Top Rated Seller (TRS) ? Then, you cannot leave any negative / neutral feedback for the first 7 days.
This gives time for busy sellers to work out a resolution with their buyer, when a buyer contacts them.
You have done this and no result, so wait out the 7 days and if you wish, can then leave appropriate feedback.
If it's not a TRS seller, then Try the ' Leave Feedback' link at the bottom of the page.
Link here:
https://www.ebay.co.uk/fdbk/leave_feedback
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