The activity display may not be available for certain third-party network interfaces: If you are using an additional network interface on your computer which has not been made by Apple, e.g. a USB-to-Ethernet adapter, it might not be offered for monitoring at the network part of the System Activity monitor item, although it might be available for the Network monitor item.
Workaround: This problem is caused by missing functions in the macOS drivers for specific network interface hardware, for example Apple’s “Skywalk” Legacy Ethernet driver. Such drivers only implement basic features to connect to a network, but don’t collect the statistical information that is necessary for high-speed activity monitoring. Although you can monitor the network detail statistics of this interface with the Network item, you cannot monitor its live actions with the System Activity item. This is the correct and intended behavior. System Monitor will automatically start offering the affected interface for activity monitoring again when the respective macOS drivers are working correctly.