Validation in the Model not working

Sorry i dont really understand, Do you mean that the error is here :

protected $table = 'walking_stats';
protected $primaryKey = 'day_walked';
protected $returnType = 'object';
protected $allowedFields = ['day_walked', 'cal_burned', 'miles_walked', 'duration', 'mph', 'shoe_id'];
protected $db; ===> HERE ?
protected $builder;