The free Fantasia potion used to be part of the Veteran Rewards that you would get after being subscribed for a certain amount of time.
The Veteran Rewards system was changed with Patch 4.1, and now you instead get a free Fantasia potion after finishing the quest The Ultimate Weapon.

The few people who had already finished that quest before the change went live, but had not yet qualified for obtaining it via Veteran Rewards, those would unfortunately fall between the cracks and not get a free Fantasia potion.

https://na.finalfantasyxiv.com/lodes...f0d5dc926e72db