Call the bookmark toc and click Add. This creates a bookmark before the title, named in such a way that KDP's automatic conversion tools will recognize it as the start of your Table of Contents.

Go to the Table of Contents section and right-click the entry that contains the error, then select Toggle Field Codes from the context menu. You will see the field name behind the bookmark, but ...

How to Create a Table of Contents Automatically in Word Microsoft Word can automatically create a table of contents (TOC) based on the headings in your document. To use this feature, you first ...

Fix Bookmark not Defined in Microsoft Word You can fix Bookmark Not Defined or Error, Reference source not found problems when you update a field that has a broken link to a bookmark in Word.