There could be several reasons why you cannot pay.
The seller may be new and needs to accept the payment or confirm their email address.
They may need to relink their eBay and PayPal accounts.
The seller may have not set up the listing correctly. If so, the seller must correct the mistake.
I am not sure the transaction will succeed now though. The seller may need to cancel the transaction, correct the mistake, and re-list for you to buy again.
The seller may have blocked you and not realised it. Sellers can block buyers for several reasons:
https://offer.ebay.co.uk/ws/eBayISAPI.dll?BuyerBlockPreferences
If you don’t meet the seller’s buyer requirements you would have been told which one, so you could correct it.
As a last resort, if there is an option to cancel your payment, do so and buy from another seller.
