Troubleshooting Missing 20% Discount on Upgrade Page

Hi, my discount offer on my upgrade page isn't showing. I want to offer a 20% discount on a one-time payment. I think I need to tick the one-time default box at the bottom of this offer section (attached screenshot), but it's greyed out. Anybody got any ideas? Here's the link to the upgrade page: https://www.writelikeyoumeanit.com/upgrade