Add import os statement

app.py

+import os
 from flask import Flask, render_template, request, redirect, url_for, send_from_directory
 from flask_sqlalchemy import SQLAlchemy
 from flask import abort, jsonify
-import os  
+
 
 app = Flask(__name__, static_folder='static')
 app.config['SEND_FILE_MAX_AGE_DEFAULT'] = 0