To be able to boot Armbian SD card image and install Armbian to eMMC, use one of the methods to boot into upgrade mode. If you have most recent board revision (1.4 ->) then you only need to quickly press the function key (middle) 3 times in 2 seconds. Release and boot will proceed from SD card. Then use nand-sata-install utility to flash to eMMC.
Blue color line: Tool Pin TXD, orange color: Tool Pin RXD, black: Tool Pin GND