obj_rsp       :=json.json(content_resp);
       

        v_status      :=json.json_ext.get_string(obj_rsp, 'status');
        v_req_no     :=json.json_ext.get_number(obj_rsp, 'data.requestNumber');
        --
        /*v_resp_code  :=json.json_ext.get_string(obj_rsp, 'validationResults.Code');
        v_resp_desc  :=json.json_ext.get_string(obj_rsp, 'validationResults.details');*/

The values are inside an array. So you need to get the first element of the array, then the values from that.

There are various ways you can do this. e.g. this uses JSON_table rather than PL/SQL objects:

select * from json_table (
  q'!{ 
    "Status": null,
    "RequestNumber": null,
    "validationResults": [ 
      { "code":"E1000", "details":"Validation Error" }
    ]
  }!', '$.validationResults[*]'
  columns (
    code, details
  )
);

CODE     DETAILS             
E1000    Validation Error   

Note that the example JSON is invalid: the values need to be in double quotes. I'm guessing this is a copy-paste error. If that's the data you're getting then you'll need to fix this before parsing the JSON.