"I am writing a book about wine" is accepted. There doesn't seem to be a report in the system for your answer, which means we can't see what you actually wrote, so we can't tell you what might have been the problem. It's always a good idea to use the Report button if you're fairly sure your answer was correct.
I am a little confused about the translation into English. The course suggests that the correct answer is I have been writing a book about wine, which is in past the past tense. However, I believe that 'Pisu knihu' is in the present tense. Can someone clarify please because this is not the only instance of this anomaly in the course. Thank you.