When programming and tracing, you must specify the target device or devices. This can be done using the device serial number or traits.
Use the nrfutil device list subcommand to list them. For example:
$ nrfutil device list
1090163479
product J-Link
board version PCA10056
ports COM16, vcom: 0
COM17, vcom: 1
traits devkit, jlink, seggerUsb, serialPorts, usb
Found 1 supported device
If you run nrfutil device list --json-pretty, you will more detailed information in the JSON format.