Forums like MyAbandonware or Archive.org are home to preservationists who have uploaded the game client. However, they rarely provide keys. Here, the "exclusive" aspect comes from community giveaways. Join Discord servers dedicated to "Lost FPS Games." Occasionally, developers or old journalists will drop a batch of unused exclusive keys to celebrate the game's anniversary (May 7th).
If none of the above resolves the issue, reach out to (support@777studios.com) with the following info: your account email, the activation key (masked if you’re uncomfortable sharing the full string), and a screenshot of the error. 7554 game activation key exclusive