I am pretty deep into this now and operating at about 50% understanding. We have a Raspi 3B+ running Ubuntu 20.04 for Raspi. We have a Teensy 3.7. They are connected together with a USB cable. I know that the Teensy is getting power.

This is the rule that I am being told to use:

KERNEL=="ttyACM?", SUBSYSTEM=="tty", ATTRS{idVendor}=="16c0", ATTRS{idProduct}=="0478", ATTRS{serial}=="000F0B8C", MODE="0666" SYMLINK+="linobase"

Not sure what udev rules do exactly

I have the feeling the the udev rule is not "matching" and that's why it is not creating the symlink. But I really am not at all sure. But honestly I don't really understand udev. I am just kind of plowing through with "common sense' which is not working for me.

Cable?

I was also suspecting it might be the usb cable? As I understand it many usb cables are "power only". However, the problem didn't go away when I used a "known good" data USB cable.

Code 43?

There may be an esoteric problem known as the "Code 43" problem that is in play here - see pjrc.com/teensy/troubleshoot.html. Anyone know anything about it?