If you are trying to do it directly from the Reply or Fast Reply submissions, I believe you have to be
at-least 'Customer' status or have Donor Forum access - at release, this feature was a 'perk' for users who meet one or both.
I did a brief search through all of the site-updates by Kevin and Paul since it was released and I don't see any mention of it being fully released to all of the public yet.
This post is from 2020, I don't believe it has changed - but I could be wrong
https://forums.d2jsp.org/topic.php?t=83903674&f=28My thought is perhaps you had made a purchase of FG, giving you access to the feature, and then it lapsed (been longer than 30d since purchase) so the access to the feature was revoked?
Hope that helps figure out the issue you might be having.