PM2 Start Appname Returning Error Cannot Find Modules in Ubuntu 18.04 LTS

This is my first nodejs app and when I try to run the app using PM2 then app start and immediately it’s become to errored and pm2 logs showing this

/root/.pm2/logs/app-out.log last 15 lines:
/root/.pm2/logs/app-error.log last 15 lines:
1|app      |     at Module.load (module.js:565:32)
1|app      |     at tryModuleLoad (module.js:505:12)
1|app      |     at Function.Module._load (module.js:497:3)
1|app      |     at Object.<anonymous> (/usr/local/lib/node_modules/pm2/lib/ProcessContainerForkLegacy.js:28:21) code: 'MODULE_NOT_FOUND' }
1|app      | { Error: Cannot find module 'express'
1|app      |     at Function.Module._resolveFilename (module.js:547:15)
1|app      |     at Module.Hook._require.Module.require (/usr/local/lib/node_modules/pm2/node_modules/require-in-the-middle/index.js:61:29)
1|app      |     at require (internal/module.js:11:18)
1|app      |     at Object.<anonymous> (/var/www/whatsclone/app.js:18:17)
1|app      |     at Module._compile (module.js:652:30)
1|app      |     at Object.Module._extensions..js (module.js:663:10)
1|app      |     at Module.load (module.js:565:32)
1|app      |     at tryModuleLoad (module.js:505:12)
1|app      |     at Function.Module._load (module.js:497:3)
1|app      |     at Object.<anonymous> (/usr/local/lib/node_modules/pm2/lib/ProcessContainerForkLegacy.js:28:21) code: 'MODULE_NOT_FOUND' }

I have tried sudo pm2 update | npm install | npm install express

Nothing fixed for me. Please help me

node -v
v8.10.0

npm -v
3.5.2

Thanks