Which version of the TP-link TL-WN821N are you using?
v2 uses the AR9170usb drivers
v3 uses the ath9k_htc drivers
Eitherway, make sure you download the firmware and put it in the right place.

For the TP-Link - TL-WN821N v3 you need ath9k_htc drivers
http://linuxwireless.org/en/users/Drivers/ath9k_htc

Hi,
I'm planning to buy a WLAN USB Adapter one of the following days:
I got the choice between the WUSB54GC en the WN821.
From the Linksys, I am certain that it is fully supported by BT4 R2....

Ive got the same error, does anyone know how to fix this?