Lots of errors with des, de, de l', de la. Duolingo is not a reliable application to learn French!
It was quite good a few years ago when I went through it (already knowing French), and I expect it is not much different now.
You are a little short on specifics, but if you do find examples of incorrect usage, use the button at the bottom of the page in question to suggest a correction. The whole course has just been updated, so there are bound to be typographical errors, etc., that have not been caught yet.