Trouble migrating from API v1 to v2

Hello! I have this little code that can search dropbox file from slack,

I have been trying to migrate to API V2, but can't get it work. 

Don't know what the problem is.

# encoding: utf-8
import time
import os
import json
from flask import Flask, request, render_template, url_for, json, jsonify
import csv
import dropbox
import requests
import sys
from slackclient import SlackClient
 
reload(sys)
sys.setdefaultencoding('utf-8')
 
app = Flask(__name__)
 
SLACK_WEBHOOK_SECRET = os.environ.get('SLACK_WEBHOOK_SECRET')
 
@app.route('/', methods=['POST'])
def index():
    if request.method == 'POST' and request.form.get('token') == SLACK_WEBHOOK_SECRET:
        username = request.form.get('user_name', 'null')
        text = request.form.get('text', 'null')
        channel = request.form.get('channel_name', 'null')
        
        print channel
        print username
        print text
        
        dbx = dropbox.Dropbox('dropboxtoken')
        metadata, f = dbx.files_download('/inventory.csv')
        out = open('inventory.csv', 'w')
        out.write(f.content)
        out.close()
        
 
        with open('inventory.csv', 'rU') as csvfile:
            readCSV = csv.reader(csvfile, delimiter=',')
 
            
            for i, row in enumerate(readCSV):
                for j, column in enumerate(row):
                    if text.replace("item:", "") in column:
                        print ' '.join(row)
                
                        payload = {"channel": "#"+channel, "username": "ROBOT-1", "text": "您查詢的是::point_down:\n"+' '.join(row), "icon_emoji": ":robot_face:"}
                        r = requests.post('webhookURL', data=json.dumps(payload))
                        print (r.url)
 
 
if __name__ == '__main__':
    app.run(host='0.0.0.0')