This is not an error but a notice.
This message usually props up when you are try to install an update. The update can be of a plugin, core or any theme. This is a way of WordPress telling people that the site is undergoing some update.
WordPress uploads a file .maintenance to root when doing an update. It gets stuck in maintenance mode due to slow server, or not enough memory problems. Or if you are trying to update many plugins at once, or if the theme is too big for WordPress to handle, it may go to maintenance mode.
After completing the update WordPress will automatically delete the maintenance file. If it does not, then we have a problem.
In this article I am going to show you how we can solve the unavailable due to maintenance issue.
First, just FTP to your site, find the file at document root and delete it. Don’t do it quickly though, otherwise problems may arise. Wait for 5 – 7 minutes and then do it.
If that does not solve the problem, then open your wp-activate.php file. This file is found in the document root. Find the line
and replace it with
If the problem keeps repeating then you are stuck with a slow host. In that case, change the hosting.