Translation:My daughter is 17 years old.
"Shi" is supposed to be used with nouns, not verbs, or closer to this case, prepositional phrases.
It goes after a number and means "years old". That's all I can tell you.
I just tried “my daughters are 17 years old”; as we should allow for the circumstance of twins. It was not accepted, so I have suggested it
They have to program it and make it better (like writing a story)
it can only accept as many words as they put as 'correct answers' so duolingo improves over time. In a year the chinese course will be much better, and those answers will likely be accepted.
-this is also why new courses will definitely be bad. (at least worse than older courses)
if you want to help the system, tell them what should be accepted by reporting your answer and putting it in the comments ^_^