This error is common in WordPress. While updating themes, plugins or core WordPress creates a temporary .maintenance file in your WordPress installed folder. After a successful update, the temporary file deleted automatically. If there are some errors happens in the update process, you will see the error. The slow server is the reason behind this error.
How to fix WordPress scheduled maintenance mode
1 – Login to your website using FTP
2 – Find .maintenance file
3 – Delete the file
4 – You are done
The files may be hidden, so you need to check show hidden files in your FTP client.