Why is it I click province quest it says this function is enabled when the emperor opens national chapter?

This message means that the buildings that control the quests have not been placed by the current emperor of your province. Every Saturday there is the battle for the throne where a new emperor is chosen. If someone from your province wins then they will be able to place the quest buildings and assign the players responsible for them.

In a Kill Event its possible for someone from another province to win, in which case they may not build them at all and you will have to wait a week for the next opportunity.

It is also possible that the buildings are placed and you need to restart your game as it can glitch a little.