Why can't I offset my flight?

Hi all,

I've recently started doing the green tier and I've been trying to offset my flight that's booked for 12th Feb. When I go to offset the flight I can see the flight but I don't see the option to offset.

Any ideas?

Edit: So I called Qantas FF and been told I need to offset the flight when I book it so nothing can be done. I don't understand why they have the option "offset upcoming flights".

Ps. I booked the flight with flight center if anyone is wondering.

Update: you could only do it when booking the flight not after.