What am I missing? OnOff SHIM fail

I just setup a system with the OnOff SHIM. When I press and hold the button, the red LED lights and that’s all it does. The RPi never shuts down. What am I missing here?
I’m using Raspbian 4.14.98 (Feb 12). on a RPi 3B. The curl command worked flawlessly and installed the software. It then asked me to reboot - which I did. I also have the USB power connected to the SHIM USB input. After the reboot finishes, I press the button and hold for 1 sec -and- nada, zilch.
Is there something else I’m supposed to do?