@GNaFF and
@kzxy46ry
This issue will come up occassionally. It does not depend on quests etc.
As
@GNaFF has stated, you will need to upgrade your team (enough to beat that stage without any hacks) to bypass that notification, before it pops up again on a later stage.
In addition, I do not read Traditional Chinese, thus it is hard for me to understand certain mechanics like synergy etc.... and also the description of hero abilities. LOL.
As the game is fairly new, just follow the Facebook Forum to gather knowledge as to which heroes are the meta.