The "Briefly Unavailable for Scheduled Maintenance" error message in WordPress typically appears when WordPress is updating itself or when you're updating plugins, themes, or WordPress core.
When WordPress proceeded with an update and automatically created a temporary file titled .maintenance Usually, this file gets automatically deleted after the update is done.
Method 1: Delete the .maintenance file
(i) To fix this, go to your Cpanel.
(ii) Click on File Manager > public_html file. Then search for the .maintenance file.
(iii) Right-click on the file and select Delete > Confirm.
Refresh your WordPress website where the error should not appear anymore.
Method 2: Manually update WordPress, plugins, or themes
Rename the "plugins" folder to something like "plugins_old." This will deactivate all your plugins.
Visit your website. If the error is gone, you've identified that a plugin caused the issue.