[PluginReference] private static Plugin ServerRewards;


private class TestScript : MonoBehaviour
{
    ***

    private void Awake()
    {
         var a = ServerRewards?.Call<object>("AddPoints", id, amount);
        Puts($"Reward = {a != null}");
    }

    ***
}

Why its not working?

P.S. - Its working on normal void out of script

Can you please explain "not working" in this context?

Function isnt working. I mean in that current situation -> its doesnt adding points

A plugin reference is set when the other plugin has loaded, so it's not really possible for it to be static. You'll likely need to rework your code a bit.