i had created ecommerce mern app . It works locally but after deployed in heroku it cant able to fetch data from server

It showing :

"message":"Operation `products.countDocuments()` buffering timed out after 10000ms",
"stack":"MongooseError: Operation `products.countDocuments()` buffering timed out after 10000ms\n    at Timeout.<anonymous> (/app/node_modules/mongoose/lib/drivers/node-mongodb-native/collection.js:149:23)\n    at listOnTimeout (internal/timers.js:557:17)\n    at processTimers (internal/timers.js:500:7)"}

link for the code in github : https://github.com/vijay-guru/vshop

Anyone please help me in finding the error....