you read (past tense) = okudun
you read (present simple/aorist) = okursunuz
you are reading (present continuous) = okuyorsunuz
(okur = he/she reads)
If translating English to Turkish, "read" is ambiguous (without sound), so Duo should accept either past tense or simple present (aorist). When translating Turkish to English, "okursunuz" can only be "you read" (simple present).
I'm not a native speaker but I think it's as follows:
"[Siz] kahverengi kitabı okursunuz." = "You read THE brown book."
"[Siz] kahverengi bir kitap okursunuz." = "You read A brown book."
Siz is left out, since the suffix of the verb already indicates that the subject of the sentence is siz. The book is the object of the sentence, so if you want to say "the book" you need to use the accusative case: "kitabı".
If you're trying to say "a brown book", it's just "kahverengi bir kitap". Make sure "bir" is between the adjective and the object :)