I tried to raise an unpaid item case but it keeps telling me that the item number is incorrect

I sold an item on EBay 3 days ago but the buyer refuses to pay or to answer any of my emails, from a facebook entry I think she is based in Bratislava, Slovakia. When I tried to open an unpaid item case I kept getting the message "item number incorrect enter another number" I tried that several times but could not proceed any further so I am stuck, how can I proceed ? 

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OMG not only can't you count the days, if the Buyer was in the UK would you really post using a method suitable for value of £20. Don't you think for the value Special Delivery + extra cover might give you some protection against loss or damage as you would still have to refund the buyer in full, whether you could get any compensation or not.