The top drop-down box in the VU Meter plugin is used to connect the plugin to your external analog VU Meter hardware—specifically the VUx2, VUx4, or VUx8 units. This setting determines which physical meter receives data from the plugin. For example, if the plugin is placed on a specific channel strip in your DAW, the plugin will send that channel’s level data to the corresponding physical VU meter you select from the drop-down.

Do you have the VU Meter Setup - Plugin on a channel strip? The VU Meter Setup - Plugin must be loaded to send the data to the physical VU Meter. See the VU Meter setup guide link below.