The USB wasn't created properly, or the bootable files were not copied correctly to the USB flash drive.
If all else fails, try a different version of HDD Regenerator or an entirely different bootable environment. Some older versions may have compatibility issues with modern hardware. The Hiren's Boot CD (an older version like 10.6) is known to include a stable version of HDD Regenerator. Booting from this CD and launching the tool from its menu may bypass the "bad command or filename" error entirely.
or
The user likely formatted the USB using a FreeDOS image provided by Rufus. However, the HDDREG.EXE file was not copied to the root of the USB drive, or the AUTOEXEC.BAT file was pointing to a path that didn't exist (e.g., trying to launch from a CD-ROM drive letter that wasn't mounted).
You cannot "regenerate" the drive that Windows is currently running on. If you need to fix your primary C: drive, you must use the bootable USB/CD method. Hdd Regenerator Bad Command Or Filename
A corrupted or improperly created bootable drive is a common culprit. Ensure you are using the correct method to create it:
HDD Regenerator was built for older magnetic spinning hard disk drives (HDDs) and relies on outdated DOS environments. If you continue to experience issues getting it to run, consider using modern, highly reliable alternatives that run directly from Windows or specialized Linux boot disks: The USB wasn't created properly, or the bootable
Solved: HDD regenerator 2011 stops working. - Experts Exchange
DOS requires exact character matching. Typing hddreg instead of hr , or vice versa, triggers the error. The Hiren's Boot CD (an older version like 10
Hdd Regenerator is an older Windows utility that scans for and attempts to repair bad sectors. The “Bad command or filename” error usually appears when trying to run it from MS-DOS/Command Prompt and the executable or script cannot be found or run correctly. Here’s a concise guide to diagnose and fix it.