I am doing API Automation Testing using JSForce to connect with Salesforce Connected App to create some data like "Opportunities" , "Accounts"

This is what I use to login and to pull the opportunity records:

const jsforce = require('jsforce');
require('dotenv').config()

const {SF_LOGIN_URL, USER_NAME, PASSWORD, SECURITY_TOKEN} = process.env
const conn = new jsforce.Connection({
    loginUrl: SF_LOGIN_URL
})

conn.login(USER_NAME, PASSWORD+SECURITY_TOKEN, (err, userInfo)=>{
    if(err){
        console.error(err)
    } 

    conn.query('SELECT Id, Name FROM Opportunity', function(err, opportunity){
        if(err){ return console.error(err);}
        console.log(opportunity.records)
    });
});

I think you have to declare the requirement for jsforce, and you have to add the security token.

i guess your function conn.sobject should be called when you hit some address like this

    app.get(`/api/sessiondetails`, (req, res) => {
        console.log(conn);
        conn.sobject("Account").create([
            { Name : 'My Account #1' },
            { Name : 'My Account #2' }
            ],
            function(err, rets) {
                if (err) { return console.error(err); }
                for (var i=0; i < rets.length; i++) {
                    if (rets[i].success) {
                        console.log("Created record id : " + rets[i].id);
                    }
            }
        });
      
    });

or it should be inside the login function conn.login(), i guess they ran asynchronously so it may have no idea what conn.sobject is. with express

const express = require('express');
const app = express();
const port = 5000;
const jsforce = require('jsforce');

    const username = '';
    const password = '';
    
    var conn = new jsforce.Connection({
        // you can change loginUrl to connect to sandbox or prerelease env.
        loginUrl: 'https://login.salesforce.com/'
    });
    app.listen(3000, () => console.log(`Example app listening at http://localhost:3000`));

    conn.login(username, password, function(err, userInfo) {
        if (err) {
            return console.error(err);
        }
        // Now you can get the access token and instance URL information.
        // Save them to establish connection next time.
        console.log(conn.accessToken);
        console.log(conn.instanceUrl);
        // logged in user property
        console.log('User ID: ' + userInfo.id);
        console.log('Org ID: ' + userInfo.organizationId);
        // ...
   
   
   
    
    }); 

    app.get(`/api/sessiondetails`, (req, res) => {
        console.log(conn);
        conn.sobject("Account").create([
            { Name : 'My Account #1' },
            { Name : 'My Account #2' }
            ],
            function(err, rets) {
                if (err) { return console.error(err); }
                for (var i=0; i < rets.length; i++) {
                    if (rets[i].success) {
                        console.log("Created record id : " + rets[i].id);
                    }
            }
        });
      
    });