Translation:I was sleeping at my parents' for a week.
Your answer is incorrect! Parents is incorrect, there should be an apostrophe!
There is. "... my parents' for a week".
Maybe it's not visible on smartphone, though.
well on mobile when you have sentences like this and you have to click on the words to form the correct answer, sometimes it gives you the word without the apostrophe so you can't even type it, then it says "you have a typo in your answer" or something like that