`boards.txt` specifies the TRRS variant here: https://github.com/adafruit/ArduinoCore-samd/blob/576d6271eddd694d13313dea7125add64be0d0e0/boards.txt#L914 The directory under `variant` all lower case which prevents the Linux IDE from finding the board.
boards.txtspecifies the TRRS variant here:ArduinoCore-samd/boards.txt
Line 914 in 576d627
The directory under
variantall lower case which prevents the Linux IDE from finding the board.