The 'Not available' note marks products in the buybox that are not available (inactive).
Go to the "Product pages" tab
Expand the "Delivery information (in the buy box)" block
Scroll to the section "Not available"
Enable the configuration "Show 'Not available'"
Save your changes after editing
Tab "Product pages" => Block "Delivery information (in the buy box)" => Section "Not available" => Show 'Not available'
general.deliveryNotAvailable
Note: The note is displayed if the product is inactive (not active) – provided the "Show 'Not available'" configuration is enabled. Its appearance follows the "Layout type" in the "Basic configuration" section.