I assume the sale is

2012 Australian Open $2 Selectively Gold Plated Silver Proof -Very Scarce

see

https://www.ebay.com.au/help/buying/paying-items/paying-with-afterpay?id=5088

“These are the eBay categories where Afterpay isn't available:   Coins (all)”

The clue is on your original listing – “Afterpay may be available” rather than Afterpay IS available. Did your buyer suggest in their messages that they assumed they could pay by Afterpay and now find they can’t? (because the Afterpay option would not have been shown on their payment invoice.) Can you check their Feedback? Although you can’t see what they have actually bought, if they have bought before from sellers of coins, they would be well aware that they couldn’t use Afterpay, and may be trying to scam you in some way.

As others have already said, as a Seller you can’t send an invoice that allows them to pay in instalments. So message them and politely say that as Afterpay is not available for sales of coins, they will have to pay in full as on your invoice. If they suddenly find the money, excellent; if not, as soon as you can (I am unsure whether you sent a second invoice or not) cancel the sale with the reason Buyer Did Not Pay. Better luck next time.