Are you perhaps using the quick listing tool? If so, switch to advanced listing - perhaps that will help. I've just checked in my Sell page, in one of the play-around-with-drafts that I have set up.
Under Selling Details, beneath Format, Duration, Price, Quantity, and Private Listing, there's this:
Payment options
โ Your PayPal account email address (to receive payments): [email address field]
โ Require immediate payment with Buy It Now
When you tick the box for immediate payment required, the following appears:
Additional checkout instructions (shows in your listing)
I hope that helps you to get the option showing.
