Why it's flagged. Your file uses a font other than the usual body font, one entry per font. The factory sets everything in the book's face, so fonts in your file are ignored.
How serious. Usually Just noting; Worth a look for a monospaced font, which often means computer text.
How to fix it. In any editor:
- Computer text or code: apply the Code Block style, or put
[[code block]]at the start of the first paragraph and[[/code block]]at the end of the last. - Otherwise there's nothing to do.
When to leave it. If the font is a paste leftover, leave it. The book won't show it.