Yes, chaining MDX calculations is absolutely possible. In fact, it is virtually unavoidable for any but the most basic scripts.
Note that Transaction Count in your formula does not affect the MTD call which errs out.
Your MTD syntax will break for any cell where the current member is not a day. If will also not work if you haven't marked the Month attribute in the Dates dimension.
However, I do not recognize the wording of the error message you quoted. What system generated the message, and is it its exact wording?