How to Fix Briefly Unavailable for Scheduled Maintenance Error in WordPress
When you go to update a plugin or a theme, WordPress puts itself in a temporary maintenance mode state. When you see the “Briefly unavailable…” message, that’s a WordPress-generated notification that alerts any visitors about what’s happening with your site.
Once the update process is finished, WordPress automatically restores access to your website.
If your server has a slow response time or for some reason the update script is interrupted (or times out), WordPress will no longer be able to restore access to your website.
When this happens, WordPress generates a .maintenance file in the root directory of the installation.
With normal behavior, the update script completes and WordPress auto-removes the .maintenance file.
In the case of an interruption of some sort, this file doesn’t get deleted and the message won’t go away.
If this happens on your website while updating, you can easily fix this by manually deleting this file.