Then either you had another error or there was a bug. The solution is correct and I know from having been a course contributor before that it is marked as accepted.
Why is "du" not an accepted answer? There is no indication that it is being told to a group of people or a singular person. Both "du" and "ni" should be accepted as correct answers.
They're both accepted. But I can see you left an error report (which is great) - your report reads du dricker inte vatten, which means "you don't drink water". The sentence is in the definite: du dricker inte vattnet = "you don't drink the water".
One is singular other is plural in Swedish, but "you" is in use for both in English, that part of the test needs a bit more work i guess, maybe adding images to questions will help.