heroku build succeed but url gives application error while heroku open - node.js

Question

Counting objects: 3, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 327 bytes | 0 bytes/s, done.
Total 3 (delta 2), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----> Node.js app detected
remote:
remote: -----> Creating runtime environment
remote:
remote: NPM_CONFIG_LOGLEVEL=error
remote: NODE_VERBOSE=false
remote: NODE_ENV=production
remote: NODE_MODULES_CACHE=true
remote:
remote: -----> Installing binaries
remote: engines.node (package.json): 8.11.1
remote: engines.npm (package.json): 5.6.0
remote:
remote: Resolving node version 8.11.1...
remote: Downloading and installing node 8.11.1...
remote: npm 5.6.0 already installed with node
remote:
remote: -----> Restoring cache
remote: Loading 2 from cacheDirectories (default):
remote: - node_modules
remote: - bower_components (not cached - skipping)
remote:
remote: -----> Building dependencies
remote: Installing node modules (package.json + package-lock)
remote: up to date in 4.243s
remote:
remote: -----> Caching build
remote: Clearing previous node cache
remote: Saving 2 cacheDirectories (default):
remote: - node_modules
remote: - bower_components (nothing to cache)
remote:
remote: -----> Pruning devDependencies
remote: Skipping because npm 5.6.0 sometimes fails when running 'npm
prune' due to a known issue
remote: https://github.com/npm/npm/issues/19356
remote:
remote: You can silence this warning by updating to at least npm 5.7.1 in your package.json
remote: https://devcenter.heroku.com/articles/nodejs-support#specifying-an-npm-version
remote:
remote: -----> Build succeeded!
remote: -----> Discovering process types
remote: Procfile declares types -> web
remote:
remote: -----> Compressing...
remote: Done: 28.5M
remote: -----> Launching...
remote: Released v6
remote: https://ashishkr.herokuapp.com/ deployed to Heroku
remote:
remote: Verifying deploy... done.
To https://git.heroku.com/ashishkr.git
cfc5c8a..76b27d1 master -> master
I have also a Procfile in my project root and have like:
web: node server.js
on doing heroku config i get the following result :
MONGODB_URI: mongodb://:#ds163410.mlab.com:63410/ashishkr
i hide the username and password for the security concern but i am sure that the username and password which i have entered is absolutely correct!
in my package.json i have engines and start script also:
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
"start": "node server.js"
},
"engines": {
"node": "8.11.1",
"npm": "5.6.0"
},
Anykind of help is appreciated !!
Thanks !!