Hi @Admin@ledgerlabs ,
It's unclear from your post which error you are encountering when you try to switch back to the legacy users list experience. I tested in my tenant and was able to switch without facing any issues. After you switch to the old experience, you can refresh the page to go back to the new experience.
There isn't currently a way to disable the "Want to switch back to the legacy users list experience?" banner entirely though. If you are having issues seeing any of the users, I would recommend verifying that the "Enhanced user management" preview feature is turned on under Preview features.
Feel free to share any screenshots of what you are seeing though, so that I can better reproduce and report the issue if needed.
If the information helped you, please Accept the answer. This will help us as well as others in the community who may be researching similar questions.