The file is coming through as a 404 Not Found, and may not be the only missing file, so try downloading WordPress again, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), and delete then replace your copies of everything on the server except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings.
Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.
From the WordPress 4.8 release notes:
Marius L. J. (@clorith)
3 months, 4 weeks ago
Known Issues
Editor tools are not showing as expected in WordPress 4.8.1 – Our editor, TinyMCE,
was updated in this release, so your plugin or theme may need to be updated to reflect this change
Thank you all! Just a bad import of the site. Sorry for the topic.
