"Это наша книга."
Translation:This is our book.
I think of it as "this [unknown, genderless thing] is our book." It's different from saying "This book is ours" which is Эта книфа наша.
You have the wrong gender - Этот is masculine nominative singular and книга is feminine nominative singular. "This book" = Эта книга
I think of Это наша книга as "this [unknown, genderless thing] is our book." It's different from saying "This book is ours" which is Эта книфа наша. It's sort of a magic trick - we don't know what the thing is until we hear наша книга.
You have to load Russian into your phone. Consult your phone's provider to figure out how to do that. On my IPhone, I do that under Settings/General/Language & Region. I then can switch languages using the globe icon on my keyboard to change language. The name of the current language appears on the space-bar
Did you copy/paste your answer here? If Duo said there's a typo, there's probably a typo you're missing - although there have been a couple times when Duo has offered an "alternate" answer which is exactly the same as the answer I typed.
I wrote This is our book and got the message that my answer was incorrect
If you wrote the exact same thing as the translation, report it.