Sui (su + i) translates as "on the" for plural nouns. No matter which direction you are translating this exercise, there is no "the" in either the English nor Italian sentence. Sul and sui do not work to translate the English sentence, and adding a "the" is not a proper translation for the Italian exercise (because there is no 'l' nor 'i").
Why? Well, the only definite article in English is "the", and when used it refers to a specific thing/noun. I can only guess that the intention of the exercise is to:
1) Point out that "sheets of white paper" could be considered a general and non specific thing, thus no "the". (See Justinnnnnn's and others comments below).
2) Focus on understanding the definite article 3) The exercise is pointing out that there are multiple ways to create a meaning in both languages. Sometimes we use "the" and sometimes we don't.
My best advice is to not add words that may mirror how you are used to saying something when translating but to focus on the parts of speech/grammar and translate from there.
Yes, I made the same mistake. Of course in English 'the' makes the sheets of paper more specific but when you use articles in Italian is very difficult to grasp. However I find it very annoying, as an English speaker, when my English is marked wrong. Sometimes the 'mistake' is simply bad typing!