ResetButtonCallback is not initially registered once the client starts up, you should instead wrap it in a pcall and continue trying to until it references it.
The pcall function is basically like a try-catch. You pass in a function and arguments, if an error is thrown, it returns false and the error, otherwise true and the results of the function if any.
You can then do this to disable the reset button:
repeat task.wait(.2) until pcall(game.StarterGui.SetCore, game.StarterGui, "ResetButtonCallback", false)