The issue here is that you haven't got enough points yet.
Besides the mission (cheese plates in this case) you will also get points that reaches at least one star. As soon as you have minimum one star marked up to the left the level will be completed.
Got all the way to level 622 without knowing that. Guess always got enough points first. I did pass it several tries later, just didn't understand why it didn't accept.
@connan That same thing has happened to me as well and got confused as well.
Usually you get enough points to the first star automatically when playing and hitting the level mode target. On some levels though it's a bit trickier to get points and if you only make short words with low point tiles on these levels, this can happen.
I'm on level 641 and it has happened to me only a few times.