"I don't know the crowd."
Translation:Saya tidak kenal kerumunan itu.
I think why it's incorrect is because the word meaning of "the crowd" here is not some specific group of people ("kelompok" specifies a certain number of people), but just many random people being in an area.
Side note: "kelompok" just means group(s). If you want to say a group of people, it should be "sekelompok orang."