See this post: Librem 5 Flashing the Jail - #4 by irvinewade for what the Librem 5 should look like when using lsusb on the host computer if you successfully put the phone in serial download mode.
Even if you did not succeed in getting serial download mode, the phone should show up as something. Maybe check/change USB port on host, USB cable, USB cable orientation.
But for sure if the host computer does not see the phone as an appropriate type of USB device then communication between the host computer and the phone using uuu will fail.