fastboot cannot find partiton

请问我用fastboot烧写镜像，出现以下问题，我应该怎么解决

运行结果及报错内容

$ fastboot devices
107A11352F0 fastboot
$ fastboot flash system_a system.img -S 50M
sending sparse 'system_a' 1/29 (51196 KB)
OKAY [ 3.115s]
writing 'system_a' 1/29...
FAILED (remote: cannot find partition)
finished. total time: 3.127s
$ sudo fastboot reboot fastboot
fastboot: usage: unknown reboot target fastboot

第一个错误:FAILED (remote: cannot find partition):
system_a 分区不存在,如果想要刷system分区: fastboot flash system system.img
第二个错误:fastboot: usage: unknown reboot target fastboot :
如果是重启: fastboot reboot