I don't know what that error means. But I can say with certainty you do not need Teensy hardware connected to download or use the software. Of course, without hardware all you can do is compile programs.
Maybe something went wrong with the download? Or if you're using Windows, maybe Windows Defender or other anti-virus software made a mistake and deleted a necessary file? If just trying again later doesn't work, maybe search for into about how to clear Arduino's download cache. I know it's been discussed many times on their forum.