Problem with xamarin and android SDK

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C#

Android

Xamarin

Hi I'm trying to learn xamarin!
Now i installed my android sdk files but in visual studio i have an error says that i need to install one of the sdk files!

Severity	Code	Description	Project	File	Line	Suppression State
Error		Xamarin.Android for Visual Studio requires at least one of the following Android platform installed: 
Android 8.0 (API Level 26 - Oreo)
Android 7.1 (API Level 25 - Nougat)
Android 7.0 (API Level 24 - Nougat)
Android 6.0 (API Level 23 - Marshmallow)
Android 5.1 (API Level 22 - Lollipop)
Android 5.0 (API Level 21 - Lollipop)
Android 4.4.87 (API Level 20 - Kit Kat + Wear support)
Android 4.4 (API Level 19 - Kit Kat)
Android 4.3 (API Level 18 - Jelly Bean)
Android 4.2 (API Level 17 - Jelly Bean)
Android 4.1 (API Level 16 - Jelly Bean)
Android 4.0.3 (API Level 15 - Ice Cream Sandwich)
Android 4.0 (API Level 14 - Ice Cream Sandwich)
Android 3.2 (API Level 13 - Honeycomb)
Android 3.1 (API Level 12 - Honeycomb)
Android 3.0 (API Level 11 - Honeycomb)
Android 2.3 (API Level 10 - Gingerbread)
Android 2.2 (API Level 8 - Froyo)
Android 2.1 (API Level 7 - Eclair)
Android 2.0.1 (API Level 6 - Eclair)
Android 2.0 (API Level 5 - Eclair)
Android 1.6 (API Level 4 - Donut)


Please click here to open the SDK Manager and install an Android SDK Platform.		 	0	

What I have tried:

please help me to solve the problem!


Edit:

Also i cannot to set target framework in properties! (the target framework list is empty!)