I did that before. I already reset the it to downloaded the Surface Recovery Image and reload the factory OS which should activate with the inbuilt key , but it didn't make any sense and I had the same problemAssuming the original enterprise owner reset the device using their enterprise OS I don't think there's a way around that unless you acquire your own enterprise key.
Assuming you are not an enterprise and you want to use this for personal use. I think the way forward is to download the Surface Recovery Image and reload the factory OS which should activate with the inbuilt key. As far as I know there's no way to remove the original key but it wouldn't work for an enterprise OS.