The daily mission require watching rewarded ads. These are the optional ads that you watch to get extra hints or access daily puzzles for the previous days. 

To get an extra hint, tap on the lightbulb icon in the top right corner of the level screen and watch an ad (available when you don't have any hints).

To access daily puzzles for the previous days, go to the Daily Puzzles section at the bottom of the screen, and select an old daily puzzle you haven't completed yet.