Check if your computer recognizes the microcontroller when its connected via USB.
Make sure you are trying to connect to the right port. When Feelix recognizes the microcontroller, the vendor will be displayed.
When using an STM32 microcontroller, make sure the device is in RUN mode.
Make sure the port is not in use by another program (e.g. Serial monitor of the Arduino IDE)
If the microcontroller is not recognized, press the RESET button while the microcontroller is connected to the computer.
For Teensy more details on troubleshooting the connection with the Teensy visit https://www.pjrc.com/teensy/troubleshoot.html.
In case the microcontroller is not recognized, test it on a different USB port or computer if possible. The STM32 board uses USB2.0.
Try to restart Feelix in case the microcontroller is recognized by the computer but not by Feelix.