PHP Fatal error: Cannot declare class WPvivid_Staging_Copy_DB, because the name is already in use when updating WPvivid Backup plugin

It is because you are still using a pretty old version of WPvivid Pro, which is probably causing the problem.

So, it is recommended that you delete it and install the latest version of the Pro plugin(you can always download the latest version of the pro version from your WPvivid My Account Dashboard > Downloads). Here is also a step-by-step doc on How to Install WPvivid Plugins Pro in WordPress.

After that, reactivate the free version.