Reading file from SPIFFS in nodemcu using ArduinoJson
Your code at the top:

struct Config {
  String ssid = "";
  String pass = "";
  bool hFlag = false;
};


defines a variable "type". It's like making a new int or bool, but yours is called struct Config. 

After making the type, you need to create declare an instance of it.

struct Config cfg;


Then, you refer to the one you created:

  // Copy values from the JsonObject to the Config struct named cfg
  cfg.ssid = root["ssid"];
  cfg.pass = root["pass"];
  if (whichFile)
  {
    cfg.hFlag = root["hFlag"];
  }


Note that a particular issue with ArduinoJSON and Strings is that you may get an error like:

error: ambiguous overload for 'operator=' (operand types are 'String' and 'ArduinoJson::JsonObjectSubscript&lt;const char*&gt;')


There is an ArduinoJSON FAQ entry that states


  Most of the time you can rely on implicit casts.
  
  But there is one notable exception: when you convert a JsonVariant to
  a String.
  
  For example:

String ssid = network["ssid"];
ssid = network["ssid"];

  
  The first line will compile but the second will fail with the
  following error:

error: ambiguous overload for 'operator=' (operand types are 'String' and 'ArduinoJson::JsonObjectSubscript&lt;const char*&gt;')

  
  The solution is to remove the ambiguity with any of the following replacement:

ssid = (const char*)network["ssid"];
ssid = network["ssid"].as&lt;const char*&gt;();
ssid = network["ssid"].as&lt;String&gt;();