WordPress Editor Error

If disabling ALL plugins and switching to the default theme did not resolve the issue, then try MANUALLY updating. Download WordPress again and unzip it, and use that to copy up. 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. (NOTE: do not delete the wp-content directory or your wp-config.php file). Please read the Manual Update directions first.

Backup: If you haven’t already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. You can never have enough backups! See https://wordpress.org/support/article/wordpress-backups/