I don't like that you can't listen to it slowly, like in the other languages I learn. For me it's still so hard and what I hear is just 'seseferiufer'... I don't make a sentence of that.
In the other languages they use an automated voice, which is easier to program to slow down. There is no automated voice for Hebrew (anywhere in the world, so I hear), so all the audio is recorded by a living person. Hence there are more limitations.
They now have an option on the app (not the site) to slow down the Hebrew speech.
Was it an audio exercise by any chance? For some reason, Duolingo expects nikkud in audio exercises, and marksman a lack of nikkud as a typo, even though it's not a problem anywhere else. It's a fault of the software; I am not a native speaker, but as far as I know, it's normal to simply write יפה and for the difference in pronunciation to be derived from context.
You could try the Hebrew Nikud Keyboard Free in Google Play. I use it with my Samsung Galaxy S7 and it works quite good. When I swipe down I can switch between the standard Hebrew keyboard and this fancier one.
I thought that ספר was a fem. word. So shouldn't the answer be זאת ספר יפה instead of זה ספר יפה ?