Sometime, it happens that compilation phase get complete successfully but the binary can't get uploaded to the board.
The M0 board Flash Mode does not get activated from the Arduino IDE environment.
Activate the Flash Mode before compiling & uploading your sketch.
To do so, PRESS TWICE the reset button.
Once done, the Flash Mode is activated and the board will show itself as an USB Stick named "FEATHERBOOT" (Windows and other operating system does show a file navigation window).
Then, press the Upload button into Arduino IDE environment.
This time, the sketch will be copied to the board... and Feather M0 properly reset.
Written by Meurisse D. from MC Hobby - License: CC-SA-BY.