After updating Joomla 3.8 this error “Unknown column ‘a.client_id’ in ‘where clause'” appears when you login into back-end, and there is nothing, it means White Back-end.
There is no Menu, but if you try to select menu (click and drag) then you will see the selected text where it describes its error. It is a database not updated error, but in this situation, you can't go to Extensions and manage the database.
How to Fix Joomla Database Issues:
Login to the Joomla Website Backend.
Copy the below link, replace your domain name with www.YourWebsite.com and load.
If you are in Localhost then change it like that localhost/YourWebsite/administrator/index.php?option=com_installer&task=database.fix