Talk to Fugen to get "Advanced: The Veterans' Gala"
Clear the ★6 village quest "Advanced: The Veterans' Gala"
Forge the Flame Seal
Clear All Village Quests First
In order to get the quest from Fugen, you need to finish all village quests first, including the optional ones. Afterwards, Fugen calls you to give the "Advanced: Veteran's Gala" ★6 village sidequest to unlock the Flame Seal.