Trouble creating webook in node

I followed the steps in https://developers.ringcentral.com/guide/notifications/webhooks/quick-start to create webhook-notification.js but when I run the resulting file, it fails with MODULE_FOUND

~/ringcentral $ node ./webhook-notification.js 
internal/modules/cjs/loader.js:888
  throw err;
  ^

Error: Cannot find module '@ringcentral/sdk'
Require stack:
- /home/x/ringcentral/webhook-notification.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:885:15)
    at Function.Module._load (internal/modules/cjs/loader.js:730:27)
    at Module.require (internal/modules/cjs/loader.js:957:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Object.<anonymous> (/home/x/ringcentral/webhook-notification.js:2:21)
    at Module._compile (internal/modules/cjs/loader.js:1068:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
    at Module.load (internal/modules/cjs/loader.js:933:32)
    at Function.Module._load (internal/modules/cjs/loader.js:774:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ '/home/x/ringcentral/webhook-notification.js' ]
}

I'm new to node.js so if there is a trivial error in the example code, I might have missed it.