Too bad you didn't just post screen captures. It would have made it simpler to see.
Basically, broken links can occur in many places. The link might be from a TOC node, it might be from the Index, it might be from a link from another topic and so on. The dialog you see gives you a clue as to where the link is broken.
In this case I might assume you used the "auto-create" TOC function. There appears to be a bookmark that is being linked to. It could be the bookmark no longer exists or it could be the topic containing the bookmark no longer exists.
Does this help?
It's the AutoCreate part I don't understand. Yes, I think I probably did use the auto-create function to create the TOC. But how do I find and resolve this?
In my TOC, I do indeed have a "book" called Campaign Approval. If I right-click that and choose Properties, it doesn't show it as being linked to anything.
The book icon against that topic shows you the issue is with the TOC book. If you click on the Edit button you will get the option to point to the right place (e.g. Campaign Approval).
Hmmm. But I don't want my books (that is, the book icon) to point to anywhere; they're really just separators, like folders.
I clicked the Delete button and I got a question, "Are you sure that you want to delete the TOC reference to this page?"
I'm not sure what I'm being asked or what the results are.
If I say Yes (delete this), I think I'm okay because I don't want book icons to point to any topic. Is that right?
It's not ticked.