eBay CS is short-staffed at the moment due to the Covid-19 situation. If you try the eBay “Help and Contact” link at the top of the “Home” page then “Call-back” and “Live chat” may still be available, but you may have to search for them.
If you want to appeal the negative feedback you have received, I am sorry but I suspect eBay will not remove it as ”It is the opinion of the buyer as to how the transaction went”.
Your best option is to appeal to the buyer, and if they agree to change the feedback, send a "Feedback Revision Request”.
https://www.ebay.co.uk/help/selling/selling-seller-performance/dispute-feedback-ebay?id=4102
