Why is my Customer Behavior report empty?

The Most viewed products and Top searches lists come from the Personalization plugin. They stay empty until it is tracking:

1. Open Marketing → Marketing Home and click the Personalization tile.
2. On the Settings tab, tick Personalization enabled, Track product views and Track searches.
3. Click Save settings.

From then on, product page visits and storefront searches are recorded. Earlier visits are not filled in.

You can see the results on the Insights tab of the same page. If the tile says Not in your plan, Personalization is not part of your current plan.