The message “Briefly unavailable for scheduled maintenance. Check back in a minute.” can appear after doing an update e.g. a plugin update and the message doesn’t go away.

During the upgrade process WordPress places a file named .maintenance in the website root directory (the folder that contains the wp-admin, wp-content and wp-includes folder). If that file exists then you’ll see the message “Briefly unavailable for scheduled maintenance. Check back in a minute.”

To get rid of that message simply delete the .maintenance file.