https://books.google.com/ngrams/graph?content=write+on+a+paper%2Cwrite+on+paper+&;year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cwrite%20on%20a%20paper%3B%2Cc0%3B.t1%3B%2Cwrite%20on%20paper%3B%2Cc0

冠詞無しのほうが多いが
冠詞ありが間違いとまではいえないと思う