"We have waited a long time, but he has not come."
Translation:Wij hebben een lange tijd gewacht, maar hij is niet gekomen.
Just a general complaint but i think a sentence this long should somehow only partially penalize when you confuse only one word like 'is'. Any language exam i have taken would have done so since there are multiple things that can be graded for in this sentence.
I would in general agree especially if you translate back to English (or what ever language) and you make an english gamma-mistake like "he run" (without "-s").. But in this case, as Raahiba says, the 'is' is rather important for the past present.
It is frustrating, but then it's always gekomen zijn - if Duo let it pass here you might not learn that.