How do I enable AVB on Mac Catalina 10.15
Modern macs will generally only support AVB when you hard wire an Ethernet cable directly to a USB-4 (only USB that works) / Thunderbolt port.
Audio Video Bridging is a set of technologies that allow Ethernet to support the kind of time-sensitive data delivery needed to keep multiple connected audio devices in sync. It is not trivial.
Apple does not advertise which computers or connections support ABV. It seems most Macs today can support it, but only on a limited number of interfaces. In particular, there is no ABV support over WiFi or via a USB-to-Ethernet adapter. In fact, I cannot even get ABV to work when connected to a Thunderbolt 3 Dock.
It seems the only way to get ABV support is to connect to a built-in Ethernet port if the Mac has one or to connect via a Thunderbolt to Ethernet adapter. This is hinted at by the help message from Audio Midi Setup when you do not have AVB enabled. It says in part

See the help guide for configuration instructions if your computer does not have built-in ethernet or if it has more than one network service that can support AVB.

Unfortunately, I do not know what &quot;help guide&quot; it is referring to, but it does lend support to the idea that AVB is only supported by &quot;built-in&quot; Ethernet.