What type of manuscript should I upload?

You don't need a file at all. You can write your book in our writer.

If you already wrote it somewhere else, bring a Word document (.docx). It opens in the writer, split into chapters. It carries the structure the tools need (headings, paragraphs, italics, bold). The writer also takes plain text (.txt) and Markdown (.md). See Can I bring in a book I already wrote?

Whatever you wrote in, you can almost certainly export a DOCX:

• Google Docs → File → Download → Microsoft Word (.docx)
• Apple Pages → File → Export To → Word
• Scrivener → Compile to Word (.docx)

A little prep makes a big difference — see How should I prepare my DOCX before formatting?