How to join kingdom quest?

[tacc]

Theres a quest called "King Slime Kingdom 1" in my quest log. How can I participate?
I spoke to Town Chief Rumenus and selected "Kingdon Quest". But no matter wich "King Slime's Counterattack [X] I select, after I click "Participate" it always says "Cannot find the Kingdom Quest" after a few seconds in the lower right corner.

And yes, I know that there is a limit of 15 people and I paid attention to the Recruit Time.

And by the way. Where can I get "gathering" (necessary to get Copper Ore at Echo Cave).