Allow access to USB devices:
https://www.rcgroups.com/forums/showthread.php?2902832-Failed-to-open-serial-Ubuntu - post #4
sudo usermod -a -G dialout radimklaska
sudo usermod -a -G plugdev radimklaska
Create file:
sudo vim /etc/udev/rules.d/45-stdfu-permissions.rules
containing
# DFU (Internal bootloader for STM32 MCUs)
SUBSYSTEM=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="df11", MODE="0664", GROUP="plugdev"