Hi @Ankita_Pachauri
You have a couple of things going on....
Look at this....
POST _ingest/pipeline/_simulate
{
  "pipeline": {
    "processors": [
      {
        "kv": {
          "field": "message",
          "field_split": "\\\n",
          "value_split": ":",
          "trim_key":  " ",
          "trim_value": " "
        }
      }
      ]
  },
  "docs": [
    {
      "_source" : {
        "message": " URL : https://www.google.com\n Action Type : Response Received\n RequestDateTime : 9/24/2024 18:06:15\n ResponseDateTime : 9/24/2024 18:06:15\n ErrorCode : \n Message : \n Stage : Response Came From Web Request\n ErrLog Generated_on : 2024/09/24 06:06:15:718\n\n TrxRef.No : 111111111\n InputXML : XXXXXXXXXXX\n ResponseReceivedData : AN is mandatory0001\n---------------------------------END LOG---------------------------------------------------"
      }
    },
    {
      "_source" : {
        "message": " URL : https://www.google.com\n Action Type : Response Received\n RequestDateTime : 9/24/2024 18:06:15\n ResponseDateTime : 9/24/2024 18:06:15\n ErrorCode : \n Message : \n Stage : Response Came From Web Request\n ErrLog Generated_on : 2024/09/24 06:06:15:718\n TrxRef.No : 111111111\n InputXML : XXXXXXXXXXX\n ResponseReceivedData : AN is mandatory0001\n"
      }
    }
  ]
}
And the result...
{
  "docs": [
    {
      "error": {
        "root_cause": [
          {
            "type": "illegal_argument_exception",
            "reason": "field [message] does not contain value_split [:]"
          }
        ],
        "type": "illegal_argument_exception",
        "reason": "field [message] does not contain value_split [:]"
      }
    },
    {
      "doc": {
        "_index": "_index",
        "_version": "-3",
        "_id": "_id",
        "_source": {
          "ResponseDateTime": "9/24/2024 18:06:15",
          "Message": "",
          "InputXML": "XXXXXXXXXXX",
          "ErrLog Generated_on": "2024/09/24 06:06:15:718",
          "message": """ URL : https://www.google.com
 Action Type : Response Received
 RequestDateTime : 9/24/2024 18:06:15
 ResponseDateTime : 9/24/2024 18:06:15
 ErrorCode : 
 Message : 
 Stage : Response Came From Web Request
 ErrLog Generated_on : 2024/09/24 06:06:15:718
 TrxRef.No : 111111111
 InputXML : XXXXXXXXXXX
 ResponseReceivedData : AN is mandatory0001
""",
          "TrxRef": {
            "No": "111111111"
          },
          "ResponseReceivedData": "AN is mandatory0001",
          "Action Type": "Response Received",
          "URL": "https://www.google.com",
          "RequestDateTime": "9/24/2024 18:06:15",
          "Stage": "Response Came From Web Request",
          "ErrorCode": ""
        },
        "_ingest": {
          "timestamp": "2024-09-27T16:24:23.001941814Z"
        }
      }
    }
  ]
}
So a couple things....
First that text at the end
---------------------------------END LOG---------------------------------------------------
Does not have a : so it can not be split and fails
Also  in the middle of the message
ErrLog Generated_on : 2024/09/24 06:06:15:718\n\n 
Has 2 \ns so that also fails so I manually cleaned that up in the example above
You will need to clean that up with some processing ahead of time...
Not pretty but this works...
POST _ingest/pipeline/_simulate
{
  "pipeline": {
    "processors": [
      {
        "gsub": {
          "field": "message",
          "pattern": "\\\n\\\n",
          "replacement": "\\\n"
        }
      },
            {
        "gsub": {
          "field": "message",
          "pattern": "---------------------------------END LOG---------------------------------------------------",
          "replacement": ""
        }
      },
      {
        "kv": {
          "field": "message",
          "field_split": "\\\n",
          "value_split": ":",
          "trim_key":  " ",
          "trim_value": " "
        }
      }
      ]
  },
  "docs": [
    {
      "_source" : {
        "message": " URL : https://www.google.com\n Action Type : Response Received\n RequestDateTime : 9/24/2024 18:06:15\n ResponseDateTime : 9/24/2024 18:06:15\n ErrorCode : \n Message : \n Stage : Response Came From Web Request\n ErrLog Generated_on : 2024/09/24 06:06:15:718\n\n TrxRef.No : 111111111\n InputXML : XXXXXXXXXXX\n ResponseReceivedData : AN is mandatory0001\n---------------------------------END LOG---------------------------------------------------"
      }
    }
  ]
}
Result
{
  "docs": [
    {
      "doc": {
        "_index": "_index",
        "_version": "-3",
        "_id": "_id",
        "_source": {
          "ResponseDateTime": "9/24/2024 18:06:15",
          "Message": "",
          "InputXML": "XXXXXXXXXXX",
          "ErrLog Generated_on": "2024/09/24 06:06:15:718",
          "message": """ URL : https://www.google.com
 Action Type : Response Received
 RequestDateTime : 9/24/2024 18:06:15
 ResponseDateTime : 9/24/2024 18:06:15
 ErrorCode : 
 Message : 
 Stage : Response Came From Web Request
 ErrLog Generated_on : 2024/09/24 06:06:15:718
 TrxRef.No : 111111111
 InputXML : XXXXXXXXXXX
 ResponseReceivedData : AN is mandatory0001
""",
          "TrxRef": {
            "No": "111111111"
          },
          "ResponseReceivedData": "AN is mandatory0001",
          "Action Type": "Response Received",
          "URL": "https://www.google.com",
          "RequestDateTime": "9/24/2024 18:06:15",
          "Stage": "Response Came From Web Request",
          "ErrorCode": ""
        },
        "_ingest": {
          "timestamp": "2024-09-27T16:31:03.335552899Z"
        }
      }
    }
  ]
}