How can I fix Heroku Application Error caused by configuration property?

I have a small project that used to work on Heroku. However, after some minor changes, I get an Application Error.

When I type npm start on the Heroku console, it gives me the following error

/app/node_modules/config/lib/config.js:182
    throw new Error('Configuration property "' + property + '" is not defined');
    ^Error: Configuration property "mongoURI" is not defined
    at Config.get (/app/node_modules/config/lib/config.js:182:11)
    at Object.<anonymous> (/app/server.js:12:19)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)

I have tried to add config vars manually on Heroku but didn't help. I am not sure what's not working because everything is fine when I run it on the localhost, and I have not changed anything on the server-side. Is there any solution for this?