Unpaid item strikes (or "cancellations" as they call them now) can only be removed by winning an appeal. If it's that old I doubt the evidence needed to prove a valid reason for non-payment (especially messages between you and the seller) are no longer available. You can try at https://www.ebay.com/help/buying/resolving-issues-sellers/appealing-unpaid-cancellation-record?id=43...
Once a full 12 months have elapsed since the cancellation (not the date of the transaction--the date of the cancellation which is at least 4 days and at most 30 days later) the strike no longer counts in any of the options sellers have to block for having such strikes.
