Trouble with Sparkfun SAMD21 Mini breakout Serial USB port

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Hello everyone. I was hoping the community could help me out. I'm not sure what I'm doing wrong. I'm having trouble trying to send a command from a Linux Debian 12 computer to a Sparkfun SAMD21 Mini Breakout via serial monitor.

This Sparkfun board is connected to a few relays that simulate button presses on a separate device. Here is my current sketch.

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When I try this using the Serial Monitor on Arduino IDE on my Windows laptop it works fine.
I see the "Please Press 1 to Generate PIN" message. When I press "1" and then "Enter" I get the "Success" message sent back to me and the relays are successfully triggered.

I'm trying to reproduce this, using a linux Debian 12 device to send the command. I'm trying to use the program "screen" to send the command "1" from the Linux computer through the USB-C connector on the Sparkfun SAMD21 Mini breakout. From what I've found online this is the closest Linux program to Serial Monitor.

When I log into my linux device via ssh I can see that the Sparkfun Mini Breakout is "ttyACM0." I then use the command "screen /dev/ttyACM0 115200." I had set my baud rate to 115200 in my sketch.

However it doesn't seem to do anything. I just see a blinking cursor. I don't even get the "Please press 1 to Generate PIN" message fromt eh Mini breakout. What's odd is that I tried something similar with an old Arduino UNO and I was able to send it this command using screen from my linux device.

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Here's the link to Sparkfun's website explaining the serial USB port works on the SAMD21 Mini breakout. I notice that they mention that the "the Serial USB port isn't broken out on the Mini Breakout." I'm not sue what this means. Would this prevent me from sending it a command using screen from a linux device? Thanks for your help guys.

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Please edit your post to display code using code tags, using the <code> editor button. Do not post pictures of code or text.

You need to add a line to your code as follows, to give the system time to establish a connection before the code prints something.

SerialUSB.begin(115200);
while(!SerialUSB) delay(1);

I've never heard of "screen" but there are plenty of linux terminal programs that work fine. I usually use GTKterm. It will act like a terminal program, sending characters as you type, not like the Arduino serial monitor, where you enter a line of data and hit a "send" button.

This comment:

Also, the SerialUSB port isn't broken out on the Mini Breakout. If you want to use the pins 0/1 serial port, you'll need to replaceSerialcalls withSerial1.

means that you need a UART serial USB adapter connected to pins 0 and 1 to send and receive serial data to/from a linux USB port, using Serial1.read(), Serial1.print(), etc.

There are many suitable USB serial adapters, such as the FTDI Friend, or something like this one from Pololu.

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