To notify the site users about the schedule up-gradation process, WordPress creates a custom maintenance screen while updating the plugin, theme, or it’s own system files. WordPress system generates a temporary .maintenance file to the root directory of a site.
Once the update process has completed successfully, the system removes that .maintenance file and the site back to its operation usually. However, sometimes, the plugin/theme unable to complete the update process. As a result, the WordPress system unable to run the removal process of .maintenance file, and it continuously displaying the maintenance page with “Briefly unavailable for schedule maintenance. Check back in a minute” message.
Please note that the WordPress system does not allow to access any of the pages, including the admin panel, until we remove that .mainenace file manually.
To fix briefly unavailable notice, please check the following step by step instructions.
Step 01: Go to the site root directory where ‘wp-content’ folder has located.
Step 02: You will get that .maintenance file.
Step 03: Select and remove that .maintenance file.
Step 04: Next, go to the site and refresh it. I hope the site now back to its regular operation.
For more information, you can check the following Youtube Video tutorial.
Finally, I wish that little efforts of mine helps you, and please comment below if you have any questions.