I've had a similar issue not being able to import novels with more than 3 Acts. I often used the Four-Act Structure in pre-NC novels and have had to change the .docx to 3 for it to import. Once in NC, I can create a dozen acts.

How many words is the novel you're trying to import? And how many chapters does it have? As a workaround, you can split your novel in half, and then use the import inside the novel that was generated from the first half, to append the other half.