Please add: "sourceMap":true and "optimization":false under architect -> build-> options in your angular.json.

Do not forget to set these for your production configurations the other way around.

Angular 12 changed the default "ng build" to use the "production" configuration. You want "sourceMap" so try adding a "development" config in your angular.json like this

"development": {
  "buildOptimizer": false,
  "optimization": false,
  "vendorChunk": true,
  "extractLicenses": false,
  "sourceMap": true,
  "namedChunks": true
}

Target with either "ng build -c development", or change your default configuration for "ng serve" eg.

"serve": {
  "builder": "@angular-devkit/build-angular:dev-server",
  "options": {
    "browserTarget": "app:build"
  },
  "configurations": {
    "production": {
      "browserTarget": "app:build:production"
    },
    "development": {
      "browserTarget": "app:build:development"
    }
  },
  "defaultConfiguration": "development"
},

You can run the below command to update the angular cli. Hope it will fix the issue.

ng update @angular/cli@latest —-from=11 —-migrate-only

After following @colche answear still i was unable to generate source map, after finding few minuses, I got solution:

ng build --prod --source-map

You have to build with source map.