There is a warning in the guide that is directed at Reinand users, directing them make sure they are using the latest version and most recent firmware files before attempting the downgrade because otherwise the downgrader will fail 100% of the time when it gets to the NFIRM. That should not be a problem for you if you are using Luma3DS (though I would double check that you are using the latest 5.5 version). The only other thing that comes to mind is to wonder if you have ever installed anything like a modified TWL_FIRM or AGB_FIRM, as these would also potentially cause errors during downgrade, same for a downgraded MSET.
I only did everything the guide told me to do so I think I haven't modified any TWL_FIRM or AGB_FIRM or downgrade MSET.









