I didnt find anything about this in the Pine Forum or via google, just this thread on Armbian
https://forum.armbian.com/topic/7783-cant-install-pine-a64-l...
Do you have any more information or a link where to follow up on this? Skimming the threads in the forum
https://forum.pine64.org/forumdisplay.php?fid=67
has more then one person saying that they managed to boot from the EMMC.
edit: nevermind found it
https://www.reddit.com/r/PINE64official/comments/df6yxz/pine...
edit2: Are you sure you didnt run into a problem with Armbian? Did you test it with any other OS? There seem to be more people who had an issue which sounds similar, not being able to boot from EMMC but from SDCard with armbian.