Today I created a new blank Ionic 2 project. And ran the ionic serve --lab command to preview the output. 

The output is produced in the browser, however, the preview is missing some UI stuff. And it throws the below error.

Your current PostCSS version is 5.2.17, but autoprefixer uses 6.0.8. Perhaps this is the source of the error below.

The Latest Ionic 2 information listed below.

Cordova CLI: 6.5.0 
Ionic Framework Version: 3.5.3
Ionic CLI Version: 2.2.1
Ionic App Lib Version: 2.2.0
Ionic App Scripts Version: 2.0.2
ios-deploy version: Not installed
ios-sim version: 5.0.13 
OS: macOS Sierra
Node Version: v6.10.0
Xcode version: Xcode 8.2.1

Get the Ionic software info using the below command.

ionic info

The Ionic App Scripts Version: 2.0.2 causes this problem. So downgrade the Ionic App Scripts Version: 2.0.2 to 1.3.0 or 1.3.7.

Downgrade the Ionic 2 App script Version.

Step1: 

Delete the node_modules folder of your Ionic 2 project.

Step2.

Paste the below code in the package.json file.

{
"name": "ionic-hello-world",
"author": "Ionic Framework",
"homepage": "http://ionicframework.com/",
"private": true,
"scripts": {
"clean": "ionic-app-scripts clean",
"build": "ionic-app-scripts build",
"ionic:build": "ionic-app-scripts build",
"ionic:serve": "ionic-app-scripts serve"
},
"dependencies": {
"@angular/common": "4.0.0",
"@angular/compiler": "4.0.0",
"@angular/compiler-cli": "4.0.0",
"@angular/core": "4.0.0",
"@angular/forms": "4.0.0",
"@angular/http": "4.0.0",
"@angular/platform-browser": "4.0.0",
"@angular/platform-browser-dynamic": "4.0.0",
"@ionic-native/core": "3.6.0",
"@ionic-native/device": "^3.4.4",
"@ionic-native/fingerprint-aio": "^3.12.1",
"@ionic-native/linkedin": "^4.1.0",
"@ionic-native/push": "^3.4.4",
"@ionic-native/splash-screen": "3.4.2",
"@ionic-native/status-bar": "3.4.2",
"@ionic/storage": "2.0.1",
"ionic-angular": "3.0.1",
"ionicons": "3.0.0",
"rxjs": "5.1.1",
"sw-toolbox": "3.4.0",
"zone.js": "^0.8.4"
},
"devDependencies": {
"@ionic/app-scripts": "1.3.0",
"typescript": "~2.2.1"
},
"cordovaPlugins": [
"cordova-plugin-whitelist",
"cordova-plugin-console",
"cordova-plugin-statusbar",
"cordova-plugin-device",
"cordova-plugin-splashscreen",
"ionic-plugin-keyboard"
],
"cordovaPlatforms": [
"ios",
{
"platform": "ios",
"version": "",
"locator": "ios"
}
],
"description": "Proj_Name: An Ionic project"
}


Change the Proj_Name with your project name in the above code.

Step3.

Run the below command in the root of your Ionic 2 project to install node modules.

npm install

That's all. Now you are downgraded to 1.3.0 version. Many Ionic native plugins won't work with the latest Ionic2 App Script version. So if you face any plugin installation problem, then downgrade to the working version.

Recently I tried the fingerprint plugin and it won't worked with latest version. So I downgraded to 1.3.0.