Reinstall WordPress Button Missing

First, be sure you have a recent, complete backup copy of your files nad database site — use this guide: https://wordpress.org/documentation/article/wordpress-backups/

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.