Have you formatted your sd card correctly? I'm not allowed to post links but here's an exerpt of the quick tut on this forum:
"
take a microSD card and, using the software of your choice (I used GParted) : remove every existing partition to only have unallocated space on it (note that you will lose all info on the sd card)
create a small FAT32 partition (it doesnt matter what size but for the the guides purpose use 200Mb) - that'll be mmcbkl0p1, you can label it whatever you want
create an ext4 partition on the remaining part of the card - that'll be mmcblk0p2, you can label it "rootfs" it's important that the FAT32 partition comes first and the ext4 one comes after - on the Switch, Linux will look for mmcblk0p2, the second partition, if you have scrolling boot logs and then back to RCM it means you did it wrong
"
After that you can copy a rootfs to the ext4 partition of the sd card.
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