This often happens if the amifldrv64.sys file is missing from the same folder as CBWinFlash.exe or if run without administrator privileges.
Make sure all code relevant to reconstructing the old firmware based on BIOS region dump is contained in its own function. How to update your USB firmware | MSB Technology cbwinflashzip install
(Here, "the" refers to the utility, not the command itself.) This often happens if the amifldrv64