I had similar problem. I was trying to build an angular library and the message below appeared:

Building Angular Package

------------------------------------------------------------------------------
Building entry point 'library-name'
------------------------------------------------------------------------------
Compiling TypeScript sources through ngc
ERROR: PostCSS received undefined instead of CSS string
An unhandled exception occurred: PostCSS received undefined instead of CSS string
See "/tmp/ng-itlEgm/angular-errors.log" for further details.
[12:03:38] 'compile' errored after 17 s
[12:03:38] Error: Command `ng build library-name --prod` exited with code 127
    at ChildProcess.handleSubShellExit (/node_modules/gulp-run/command.js:166:13)
    at Object.onceWrapper (events.js:422:26)
    at ChildProcess.emit (events.js:315:20)
    at ChildProcess.EventEmitter.emit (domain.js:505:15)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:275:12)
[12:03:38] 'build' errored after 17 s

I was using gulp-run to execute the command ng build library-name --prod. But, executing the command ng build library-name --prod directly on the terminal, the log error was smaller, but similar too.

The problem was in one component. The component had one styleURL (Angular component decorator), like this:

...component.css

but, the library uses scss. So, I changed the path to:

...component.scss

and I executed the command 'ng build library-name --prod'. So, the error didn't appear again.

I was using NVM v0.35.2 (allow to have more than one NodeJS version installed in my machine); NodeJS v12.18.2; Ubuntu x64 18.0.4 LTS and Angular CLI 9.1.9.

if you are using webpack, and trying to use sass-loader, also add sass

This can happen due to change in version of node. I had same issue with a NUXT app. What I did was

1. using yarn, in the root directory of your project, run yarn. But if you're using npm run npm install
2. start your project again.

This can sometimes happen if node-sass was compiled for a different version of Node.js than you're currently running (ie if you've recently changed your Node.js version). You can fix that with:

npm rebuild node-sass

I had this same issue when attempting to test an Angular library, and the issue was I had [``] instead of [] in the styles property of my component metadata.

Wrong

@Component({
selector: 'my-input',
template: `
  <input
  ... />
  `,
styles: [``],

Right

@Component({
selector: 'my-input',
template: `
  <input
  ... />
  `,
styles: [],