"Please give me two sheets of paper."
You could. Japanese is a weird language, so you could say かみを二まいください かみにまいをください or かみにまいください and it would still make grammatical sense. Out of the three, I think the first one sounds the most natural.
Standard Japanese is かみを二まいください
かみにまいをください or かみにまいください are verbal or dialects
This is a problem I find with a few other questions as well, and it's that it only accepts certain kanji but not others, much worse in this case since when I type 「にまい」, I either get the hiragana or 「二枚」, the full kanji, but it doesn't accept either of those as correct, therefore forcing me to have to write 「二枚」, and then go back to change the 「枚」 to 「まい」.
When you come across questions that don't accept kanji, hiragana, or any combination of the two, just submit an error report and your suggestions will be added to the database eventually.
This is a weird one because I am used to sticking "TE" after the word proceeding "kudasaii" or please for the lamens. "Mai TE" would sound correct but I guess it isn't.
Verbs that come before "kudasai" are conjugated in the -te form, so that's why you're used to seeing "te" before "kudasai". Nouns take the particle を before "kudasai". In this question, we have a counter, 2枚 (2 mai). Counters don't take particles and they don't conjugate, which seems strange but is correct.