The coupons are entered in the URL directly now; we no longer have a specific field where you paste coupon codes.
Authors should hand out URLs as coupons going forward.
However, if you go to
https://leanpub.com/THEBOOK/c/THECOUPON
it may work.
For example, I have a book called
https://leanpub.com/lean/
and I have a coupon called