Assuming you are booked in the correct fare class, you can only submit the request once the booking is ticketed.
To see the booking class for each segment just go to "Manage my booking". Class N, O and Q can't be upgraded, everything else is upgradable.
Strangely I've done it before by just googling a link and doing it without the invitePoints upgrade is different to a bid upgrade. Bid upgrades you have to be invited to do so. Not on all flights and not on all days. Can vary.