Be sure that Arduino --> Preferences --> Additional Boards Manager URLs contains:
Launch the Arduino app and select Tools --> Board --> Boards Manager.
Use the search field and install both the SAMD Board packages. One is from Arduino and the other is from Adafruit.
Plug in your Feather M0. Make sure the following is selected under Arduino --> Tools:
- Board: "Adafruit Feather M0 (Native USB Port)"
- Port: "/dev/cu.usbmodem14211 (Adafruit Feather M0 (Native USB Port)" - or whatever the port is.
An ELF binary file is necessary for debugging with JTAG. In order to locate the file we will enable both of these:
"Show verbose output during: X compilation X upload"
The lower window which shows the compile process has the ELF files path and name highlighted. Not this as it will be necessary when launching gdb.