>書くの練習 As you know, 練習 is a norm and 書く is a verb. So you don't need to put の between the words, and you can just connect them, without conjugating the verb. Or if you want to use の in your sentence, you should put こと in front of の like 書くことの練習, so both 書く練習 and 書くことの練習 have almost same meanings, because we can think gerunds kind of norms. I think that 書く練習 means "a practice to write" and 書くことの練習 means "a practice of writing".