This is what the help page (https://www.ebay.com.au/help/buying/paying-items/paying-with-afterpay?id=5088) says:

Occasionally you may see Afterpay shown on an item, but then it's not available as an option at checkout. This may happen because:

  • Your order total is less than AU $1 or over AU $3,000
  • You've selected a delivery address outside Australia
  • Your order includes items from excluded categories
  • You're ordering items from different sellers
  • Items in your order have different delivery methods (for example, one with pickup and one with express postage)
  • You're using an older version of the eBay app.

Was it an excluded category by any chance?

You don't have to keep repeating yourself replying to each person. Everyone can see all replies, so you only need to reply once, to one person. While you may get email replies if you've opted for that, this isn't an email discussion group.