I re-downloaded and reinstalled Addon Manager - the error persists
The Addon Manager ASKED if you want to activate a scenery and, instead of simply replying YES, you download it again ?
how to remove this error?
That message is not an error, it simply means your system hardware config has changed in some what, triggering a reactivation. If you haven't changed anything in the hardware, it might happen when updating low-level drivers (like the sata drivers) or flashing the bios.
Simply reply YES to the activation question, and the message will go away.