Do you want it to have EXACTLY 200 words per page, or do you want it to
average around 200 words per page?
I can't imagine why a publisher would insist on the first of those options,
but the second option sounds plausible. If that is what you want, just write
your novella, and then use the Word Count feature to decide how many words
you have. Then adjust page margins, line spacing, font size, etc., so that
your page count is about 1/200th of your word count. (For example, if your
word count is around 40,000 words, adjust things until your page count is
around 200.)
Fred