Could not open /dev/ttyUSB0, the port is busy or doesn't exist
What it looks like
A fatal error occurred: Could not open /dev/ttyUSB0, the port is busy or doesn't exist. ([Errno 13] could not open port /dev/ttyUSB0: [Errno 13] Permission denied: '/dev/ttyUSB0')
What it means
The upload tool could not open the serial port. The text in brackets says why: Permission denied is an access problem, Device or resource busy means another program has the port, and No such file means the board is not seen at all.
Likely causes
- On Linux, your user is not in the group that owns serial ports.
- A serial monitor, the bench page or another IDE has the port open.
- The board was unplugged or appeared under another name, such as /dev/ttyACM0 or COM4.
- A missing USB-to-serial driver (CP210x or CH340) on Windows or macOS.
How to fix it
- Linux: sudo usermod -aG dialout $USER on Debian and Ubuntu (uucp on Arch), then log out and back in.
- Close other serial monitors and disconnect the bench page before uploading.
- Pick the port again in Tools > Port after replugging.
- Install the CP210x or CH340 driver for the bridge chip on your board.
Paste your full log
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