If you did not see /dev/ttyUSB0 in the device list, and you could not see anything matching your device, you will need to make sure the serial device is connected and supported by Linux. Then try the following:

If you do not know what your serial device is called, you can do the following.

• Make sure the USB serial adapted is connected. Then run ls /dev/tty*.
• Disconnect the serial device and run ls /dev/tty* again. Did you notice something missing compared to the previous printout?
• Now connect the serial device, wait 5-10 seconds and run ls /dev/tty* again. Do you see a new device in the list?
• If you do not see a new serial device, the device may not be compatible with Linux. Run dmesg to see if it is detected. In my case it is detected, and is named ttyUSB0:

Ignore the under-voltage error on my screenshot. You shouldn't be getting that error.

• To change the serial port used in the vmodem.sh script, do the following:
• Run sudo nano /boot/vmodem/vmodem.sh
• Find the line serport=ttyUSB0 and update it to reflect the correct serial device.
  • For example, if your USB device is called “/dev/ttyAMA0”, then make sure the line says serport=ttyAMA0.
• Save file by pressing CTRL + o, then ENTER to save under the same name.
• Exit editor with CTRL + x.