With just this snippet it is hard to tell what went wrong, this command copies two precompiled bitsandbytes dll files from the repository to your virtual Python environment. The error would occur if the source files are missing or the target folders don't exist.
- Make sure the previous steps executed without error.
- You are running the command in the right directory (kohya_ss)
- Verify that kohya_ss\venv\Lib\site-packages\bitsandbytes\exists
I don't have access to Windows, but the bmaltais/kohya_ss Github repository contains different instructions for setup. This 3 months old guide might just be out of date already. See if this makes more sense, https://github.com/bmaltais/kohya_ss#windows