@Timbo is right. I executed this command and it success every time.
nvram set ssh_en=1 & nvram set uart_en=1 & nvram set boot_wait=on & nvram set bootdelay=3 & nvram set flag_try_sys1_failed=0 & nvram set flag_try_sys2_failed=1
nvram set flag_boot_rootfs=0 & nvram set "boot_fw1=run boot_rd_img;bootm"
nvram set flag_boot_success=1 & nvram commit & /etc/init.d/dropbear enable & /etc/init.d/dropbear start
Taken from a chinese youtuber that attempted to flash RB03.