Dear Marti and William
Thank you for your reply.
I tried using "--bossac-port", but still have problem about "-o" option.
I checked my bossac option by
$ ~/Zephyr_191116/zephyr-sdk-0.10.3/sysroots/x86_64-pokysdk-linux/usr/bin/bossac --help
Usage: bossac [OPTION...] [FILE]
Basic Open Source SAM-BA Application (BOSSA) Version 1.7.0
Flash programmer for Atmel SAM devices.
There is no "-o" option for my bossac.
My environment is Ubuntu 18.04 LTS.
I have found this.
Adafruit Trinket M0 Bossac Offset is Wrong #16052
For Adafruit Trinket M0, the address is specified using "-o 0x2000". However, because my bossac does not have "-o" option, the problem occurs.
It seems that I have to update my environment to have newer? bossac version.