boot on SD disk and fastboot


I flashed my android .img to SD card and insert into beaglebone black. For most of the tutorial, I learnt I need to press the boot button before I power on the board, and then wait for a couple seconds for the LED to light up. However, when I did so, the leds never light up. However, if I press the boot bottom after I power on the board. It is booting andorid. Is there anything wrong with my u-boot?

Another question, I am trying to flash andorid to emmc memory. However, when I type in fastboot. It’s saying command not found. How should I try fastboot?