"Raj has written a book."
Translation:राज ने एक किताब लिखी है।
I agree. I feel 'राज ने किताब लिखी है' should be accepted. Both the sentences (with and without एक) sound natural and can be used to describe the fact that Raj has written a book. Not accepting this sentence is limiting oneself to just narrow translations (a very common annoyance across Duolingo).