"You read the brown book."
Translation:Kahverengi kitabı okursunuz.
"okusun" should be okursun in the simple present (aorist) tense. There is an -r- after the verb stem and before the personal suffix (okurum, okurlar, etc)
"okusun" is a real word, but it's a 3rd person imperative (something like: "may it be read"). But Duo hasn't covered that at this point in the tree, so don't stress if you don't understand.
Full conjuagation chart for "Okumak"
I initially thought this was wrong. But after reading the comments I see that 'You read' is present tense. My mistake was that I thought 'read' was past tense, so I thought it should have been 'okudun'. If there is a way to reword this it might void the confusion of past and present tense of read being spelt the same?
Past tense should be accepted as well because the English sentence is ambiguous. Perhaps there was another mistake elsewhere in the sentence. (Duo sometimes over-corrects.) Or maybe you had a missing alternative. Either way, you're correct- past tense is perfectly fine here, even though it's not the default answer :-)