Amazon Bought Game / Serial Key Invalid

[Cynthentic]

Hello, I am recently attempting to return to M&B, returning after 3 years. Back in 2016, when I bought the game, I bought the Napoleonic War DLC from Amazon, and only that. I never bought M&B Warband but at one pointed I was able to play the warband campaign, but not multiplayer, but I could play multiplayer on Napoleonic Wars. After coming back after many years and re-installing the demo version of M&B Warband and NW from the taleworld website, I attempted to use the serial key that amazon gave me, but it no longer worked. The key is a 4x4 key (XXXX-XXXX-XXXX-XXXX)

Any help would be greatly appreciated as I am quite excited to get back into the game 

edit: The message that pops up is "Activation Denied" not "Incorrect Serial Key"