I'm having a problem with Bulk Write API in Zoho CRM Plus Contacts Module

I'm having a problem with Bulk Write API in Zoho CRM Plus Contacts Module

Hi . I have some questions about integrating REST API with my Nodejs App. 

I'm new to Zoho and haven't used it before. The current client I have is using Zoho CRM Plus and he is requesting a csv file upload which he is going to do that an automatic process everyday , which I'm doing to achieve this by doing a cron job in my server and executing bulk write api functions. 

I've already created a developer app and got the refresh token with this scope : ZohoCRM.bulk.ALL,ZohoCRM.modules.contacts.all,ZohoFiles.files.ALL,ZohoCRM.org.ALL,ZohoCRM.settings.fields.all

I can insert records manually, get records and upload file then user FILE ID in Bulk Write Job. 

However when I want to do a Create a Bulk Write Job  I'm having problems with the response. 

POST : https://www.zohoapis.eu/crm/bulk/v2/write
data : 
{
  "operation": "update",
  "callback": {
    "method": "post"
  },
  "resource": [
    {
      "type": "data",
      "module": "contacts", // Contacts Module I want to insert/update/ data
      "file_id": "1437800......." , // FILE ID I"M GETTING AFTER UPLOAD
      "field_mappings": [
      {
          "api_name": "First_Name",
          "index": 5
        },
        {
          "api_name": "Last_Name",
          "index": 6,
          "default_value": {
            "value": ""
          }
        }
      ]
    }
  ]
}


 
There are two types of responses I get : 
{
"status": "error",
"code": "MANDATORY_NOT_FOUND",
"message": "Required key find_by is not available",
"details": {}
} and the other is about a file format problem ( Which I'm not getting when Uploading file , but only in Create bulk Write) job . I'm not sure about it and could not find anything. Even though I upload the file with .zip and only 2MB. My questions are : 1) How can map fields correctly with the CSV file I have. 2) Do you have any resource in NodeJs or PHP that used Bulk Write api so I can have a look ?