@alan-agius4 here are the steps that reproduce the error using the Angular CLI and following the documentation for adding Angular Universal:

$ ng new TestAngularUniversal --defaults=true --strict=true
$ cd TestAngularUniversal
$ ng add @nguniversal/express-engine
$ npm run build:ssr && npm run serve:ssr

The relevant error output is:

ERROR in server.ts:18:25 - error TS2345: Argument of type '(filePath: string, options: Readonly<RenderOptions>, callback: (err?: Error | null | undefined, html?: string | undefined) => void) => void' is not assignable to parameter of type '(path: string, options: object, callback: (e: any, rendered: string) => void) => void'.
  Types of parameters 'callback' and 'callback' are incompatible.
    Types of parameters 'rendered' and 'html' are incompatible.
      Type 'string | undefined' is not assignable to type 'string'.
        Type 'undefined' is not assignable to type 'string'.

18   server.engine('html', ngExpressEngine({
                           ~~~~~~~~~~~~~~~~~
19     bootstrap: AppServerModule,
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
20   }));
   ~~~~

I also encountered this after upgrading to angular 9, my workaround is similar to @bufke .

  server.engine(
    'html',
    ngExpressEngine({
      bootstrap: AppServerModule
    }) as any
  );

hope there is a more elegant way.

Getting the same error after following pretty much same commands with same --strict=true flag. However, when I recreated the same project without the flag, the npm run build:ssr and npm run dev:ssr commands succeeded just fine. So seems to be something regarding the --strict=true flag and Angular 9?

// Our Universal express-engine (found @ https://github.com/angular/universal/tree/master/modules/express-engine) server.engine(‘html’, ngExpressEngine({ bootstrap: AppServerModule, }) as (path: string, options: object) => void);

Thank you @tuananhitoct, this solved my problem.

@bskeen @rts-cwalker I got the same error. It is strict mode when --strict flag is set. Fix to work arround:

export function app(): express.Application { // --> this function return server and must change return type to express.Application. this is explanation for last line of this function.
  const server: express.Application = express();
  const distFolder = join(process.cwd(), 'dist/PROJECT_NAME/browser');
  const indexHtml = existsSync(join(distFolder, 'index.original.html')) ? 'index.original.html' : 'index';

  // Our Universal express-engine (found @ https://github.com/angular/universal/tree/master/modules/express-engine)
  server.engine('html', ngExpressEngine({
    bootstrap: AppServerModule,
  }) as (path: string, options: object) => void); // --> add here. because typeof engine function and return type ngExpressEngine is not match. reassign type for it

  server.set('view engine', 'html');
  server.set('views', distFolder);

  // Example Express Rest API endpoints
  // server.get('/api/**', (req, res) => { });
  // Serve static files from /browser
  server.get('*.*', express.static(distFolder, {
    maxAge: '1y'
  }));

  // All regular routes use the Universal engine
  server.get('*', (req, res) => {
    res.render(indexHtml, { req, providers: [{ provide: APP_BASE_HREF, useValue: req.baseUrl }] });
  });

  return server;
}