By playing through the story mode in Mortal Kombat 1, you will unlock rewards. One of the major ones is being able to play as Havik when you beat the entire campaign and reach the end credits.
If the rewards aren’t unlocking for you, then you can check out this troubleshooting article from Seeking Tech.
1. To start off, you should make sure that you are connected to the game servers. Otherwise, you won’t be able to claim your rewards.
2. If you earn the rewards in story mode while playing offline, then make sure you connect to the online servers in MK1. Afterwards, you can claim the bonuses from the Notifications menu in the Kombat Kard screen.
You can open the Kombat Kard screen by pressing the touchpad on the DualSense, View button on the Xbox controller, – button on the Nintendo Switch controller or M key on the keyboard.
3. If you still aren’t unlocking the story rewards, then you can try restarting your game.
On the Xbox Series X or Xbox Series S console, you need to manually quit Mortal Kombat 1 in order to avoid putting it in the Quick Resume queue. You can then open the game again.
4. You can also try replaying the story mode again, with the game connected to its servers, to see if doing so will unlock the bonuses.