If you see this notice in your WordPress Dashboard, you must follow the troubleshooting steps below:
Step 1:
You must delete the current Booked plugins via FTP or you can install the Advanced File Manager plugin:
Note! If you delete the plugin from the dashboard, you'll lose all of the plugin's data and settings.
Step 2:
Go to Vamtam > Install Plugins > and install Booked
It is the bundled with the theme version. Since the Booked Author stopped supporting the plugin, we'll release regularly the plugin's updates.
Check this article about the theme and plugin updates