I struggled with the same problem just a minute ago. My project was generated using the v 1.6.0 of angular-cli.
when I am going to start my ng server, then encounter the below error.
ng s
error⬇︎
module.js:557
throw err;
^
Error: Cannot find module '@angular-devkit/core'
at Function.Module._resolveFilename (module.js:555:15)
at Function.Module._load (module.js:482:25)
at Module.require (module.js:604:17)
at require (internal/module.js:11:18)
at Object.<anonymous> (/workspace/observable-app/node_modules/@angular-devkit/schematics/src/tree/virtual.js:10:16)
at Module._compile (module.js:660:30)
at Object.Module._extensions..js (module.js:671:10)
at Module.load (module.js:573:32)
at tryModuleLoad (module.js:513:12)
at Function.Module._load (module.js:505:3)
do not worry, here is a solution.
first, you need to change the @angular/cli version.
editing my package.json changing the line
"@angular/cli": "1.6.0",
to
"@angular/cli": "^1.6.0",
then now
npm update
and it works.
I hope it will help you too.
Enjoy Coding...