I talked about this exact topic very recently here, maybe check this out:
https://forum.krontech.ca/index.php?topic=755.0 Otherwise, if the Boot Fails on a newly Flashed Card, just retry to Flash it, maybe even with a Fresh Download of the Firmware File, just to make sure the File you try to Flash with is not broken or something. If that still leads to the Same Result, try a new/ other SD-Card with the Same Capacity (not completely sure about Minimum Required Size, but i think 16 or 32GB is pretty much the Smallest ones you can get todays anyways...), because Micro-SD-Cards can and will sometimes just Fail, when you Flash a new Firmware or even in everyday use, even though its extremely rare, but can happen.
If a new Flash on the Same Card with a Fresh Firmware File AND a new Flash on a New Card both will not work, its likely you have some other issue with the Camera, but i assume that should solve it (assuming you did nothing wrong with Flashing the Firmware, but what you linked is the Correct Procedure and somewhat hard to do wrong, so i think that should not be a Problem).