This error is caused by a fail WordPress version upgrade. Try a manual update.
Download the latest WordPress zip file and unzip it.
Back up your WordPress.
Rename wp-includes and wp-admin directories to wp-includes.bak and wp-admin.bak.
Upload the wp-includesand wp-admin directories from unzipped folder to your web host via FTP.
Upload files from new wp-content to your existing wp-content folder. Replace and overwrite existing files.
Upload the new version of the rest of files from the root directory to your existing WordPress root directory.
Remove .maintenance at your WordPress directory via FTP.
Login to WordPress admin and you will be given a link to URL like “http://domain.com/wordpress/wp-admin/upgrade.php”. Follow the link and instructions.
Clear your cache to see the changes if you have caching enabled.
For more details about WordPress manual update, read http://codex.wordpress.org/Updating_WordPress#Manual_Update.
