When books or pages are not valid, the TOC Composer displays the icons with a red X to indicate that they are not working properly. Broken links are easy to fix following these steps.
From the Tools menu, select Resolve Broken Links.
On the right, All Missing Topics displays a list of topics that are missing from the project.
On the left, References to Selected Topic shows all references to the topic selected on the right (All Missing Topics). When you select a different topic, the list on the left changes to show all references to it.
Broken table of contents
references are indicated by the icon.