Hm, that’s odd, might be server-specific block on that file for some reason. Without more details, it’s hard to tell.
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.
If you’d like to manually make a backup of your site first, please follow the steps at https://www.ads-software.com/documentation/article/wordpress-backups/
Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.
