Can’t see the design and build of Sample App “Todo” (Sample Xamarin app). How to fix it?

Can’t see the design and build of Sample App “Todo” (Sample Xamarin app). How to fix it?

In the design says “Some assembly references are missing. Building to restore the NuGet cache might resolve this issue.”

Error list :

Severity    Code    Description Project File    Line    Suppression State Error   XDG0008 The name "WindowsPage" does not exist in the namespace "using:Xamarin.Forms.Platform.UWP".  Todo.UWP    MainPage.xaml   1     

In the output Xamarin Diagnostic :

[I:]:                     Found Xamarin.Android 9.1.7.0 [I:]:                     Found Xamarin.Android 9.1.7.0 [I:sdk]:                  Runtime path: C:\Program Files (x86)\Microsoft Visual Studio17\Community\MSBuild\Xamarin\Android [I:sdk]:                  Framework path: C:\Program Files (x86)\Microsoft Visual Studio17\Community\common7\ide\ReferenceAssemblies\Microsoft\Framework\MonoAndroid\v1.0 [I:]:                     Tracking android devices started [D:]:                     TrackDeviceTask got:  [I:]:                     Got new device list from adb with 0 devices [D:]:                     Tracking avd started [D:]:                     avd watcher *.ini path: 'C:\Users\User\.android\avd' [D:]:                     avd watcher: got device list from avd with 2 devices [E:]:                     an error occurred loading avd `Pixel_2_API_28`, the system image does not exists [I:]:                     Looking for Android SDK... [D:]:                     ValidateAndroidSdkLocation: `C:\Program Files (x86)\Android\android-sdk`, result=True [D:]:                     ValidateAndroidSdkLocation: `C:\Program Files (x86)\Android\android-sdk`, result=True  

Screenshot : Problem Screenshot