To help find answers you could clarify some points first. Are you absolutely sure first that you have double-checked the instructions and you are doing them correctly? The error you mentioned FiNDTR: Cannot open md5.lst seems to indicate that you might not be executing the cmd and fastboot command on a correct path / folder.
Make sure especially
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You are using Windows 10 20H2 or later. Check this by going Settings > System > Details / info (or something like that) > Version
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You have correctly installed fastboot drivers and there’s Sony sa0114 ADB Interface Driver on Device Manager.
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You have extracted Sailfish OS image and Sony software binaries into same folder. So you should basically have all these in the Sailfish image folder.
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You are running the cmd on correct path. Make sure you are on the Sailfish image folder with all the necessary files extracted.
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Double-check the instructions and make sure you are really understanding each step.
Otherwise this might be a specific issue. It really helps if one specifies as many details as possible when it comes to asking help or reporting a bug. We need to know what have you already tried, how you are doing it etc. if you get my point.