How can I download script editor?

The Script Editor is installed by the operating system in the /Applications/Utilities folder, which is the same as Dock : Launchpad : Other. Script Editor does not exist as a standalone download from Apple.

There is no stand alone Script Editor.app to reinstall.

Reinstalling the macOS is a good solution.

The Script Editor is in the Utilities folder.

If the Script Editor is not in the Utilities folder, then you have to reinstall the OS.