What is the reason for this error?
It is incorrect to call a suspension due to scheduled maintenance an "Error". However, during the WordPress update, the necessary server files are downloaded, decompressed, and installed to complete this process. Apparently, the site is entering maintenance mode and, to inform you of this, it will display a message: "Not available due to scheduled maintenance. Please check the site again in a few minutes".
This message usually disappears after a few seconds, maximum one minute. In some cases, it does not disappear so quickly, and that is due to the slow response of the server, memory problems, and the interruption of the update script. A stop occurs when WordPress does not complete the maintenance mode to restore your website normally.
How to solve this blockage on your website?
The first thing you should check is that your website has been successfully updated to the latest version of WordPress. If the update is interrupted or fails for some reason, it can cause problems. These problems force you to manually update WordPress.
After updating the CMS, the .maintenance file is removed from the root folder of your WordPress back office. You can do this using FTP client or command line.
Removing the .maintenance file using FTP client
WordPress creates a maintenance file during update processes to indicate that your website is in maintenance mode. If you do not remove this file from the server, your website will remain in maintenance mode, preventing users from accessing the website.
With FTP client installed on the website, in this case, you can use Filezilla. You must go to the Server-> Force show hidden files menu, located in the top menu bar. Once you find the file, delete it. Later, when WordPress performs another maintenance, it will create this file without errors or maintenance problems.
Removing the .maintenance file from the command line
Our goal is to remove the .maintenance file from the root directory of WordPress as in the previous case. For that, we use the following cat command.
$ cat .maintenance
This command returns the following expression:
<?php $upgrading = 1490382161; ?>
After deleting the file, you must use the following command lines or short codes.
$ cd /var/www/html/ $ ls -l .maintenance $ rm -v .maintenance