KDP's content review flags trademarks in four places, and the rules are different in each.
Single titles: free
The title of one book is not a trademark. You can publish "The Long Road" even though dozens of books carry it. What KDP blocks is a title that reproduces a registered brand, such as a book called by the name of a famous product, or one that implies affiliation with a franchise.
Series names: a brand
The moment you publish book two under a series name, that name works like a brand and can be registered as one. Several authors have registered their series names; several more have received complaints for using one that was already registered. Before you commit to a series, check the name in class 16 and 9, and in 41 if you plan audio or adaptations.
Pen names: a brand, and occasionally someone else's
A pen name that matches a living author's name or a registered author brand is the most common KDP name problem I see. Estates and publishers register author names. Check the pen name as a brand, in the same classes, and search it as a plain web query too, because an established author with the same name will cause reader confusion long before any complaint.
Words inside the title: the trap
This is where most KDP trademark blocks land. A word in your title or subtitle that is a registered mark in class 16 or 9, used in a way that looks like a brand. Names of games, apps, diets, methods and products all show up here. Using the word descriptively ("a guide to playing chess") is fine. Using it as if it were your brand, or in a way that suggests the owner endorses the book, is not.
Run the check
Paste the title, subtitle, series name and pen name into the checker above with KDP selected. It sets the classes to 16, 9 and 41 and pulls out every phrase that looks like a brand. Anything live and close in those classes is worth changing before you upload, because a KDP block after publication means an unpublished book, a support ticket and a wait.