So the warnings applied are for mixins, configs, and commands. For the case of mixins, it seems like the class that it is applied to cannot be found. However, since the mixin is most likely marked as optional, it returns a warning in the game instead of erroring. The config warning is either because the config is not present. In that case, it just generates the config. For commands, it means when running the command, the argument being provided could be interpreted in many different ways. Though, it likely won't cause an effect on you if you are playing in a singleplayer world.