You can’t set a hard reference to a level actor from an actor, your player controller, that doesn’t exist in the level yet. You need to use a soft reference through TSoftObjectPtr.
TSoftObjectPtr<AActor> Reference;
The above code should work for what you’re trying to do. However, you should reconsider setting references this way since the level actor only exists within that level. It would be better to pass the reference from your level blueprint to your game instance or game state, then retrieve that reference from your player controller.