Why it's flagged. A paragraph looks like a chapter or section title (bold, larger, or "Chapter 3") but carries a body style. The factory finds chapters by Heading styles, so it can't see this one. It may be missing from your contents, or run into the chapter before it.
How serious. Worth fixing when your file uses no Heading styles at all; usually Worth a look when it does.
How to fix it. In any editor:
- Select the line and apply the Heading 1 paragraph style for a chapter title (Heading 2 or 3 for sub-heads).
- Remove any bold or larger size you added by hand to make it look like a heading; the book sets its own.
- Upload again and choose Inspect again.
When to leave it. If the line really is body text that happens to be bold, such as a run-in label, leave it. In the full report, correct as is stops it being flagged.