The project is missing Android SDKs required for building

I had the same issue... and Tools -> Android -> Android SDK Manager was greyed out for me.

In the end I needed to go to Tools -> Options -> Xamarin -> Android Settings and then make sure that the "Auto Install Android SDKs" was ticked. Once I ticked this and then double clicked on the error the install window suddenly appeared.

Per teapeng's answer, you may need to restart Visual Studio for this change to take effect.

Visual Studio includes an Android SDK Manager that you use to download Android SDK tools, platforms, and other components that you need for developing Xamarin.Android apps.

See Tools -> Android -> Android SDK Manager.

Under Platforms you'll find various Android versions for the emulator and under Tools you can install various tools.

Also

See Tools -> Options -> Xamarin -> Android Settings

There is a folder path called Android SDK Location, if it's empty you can browse for it yourself to find it.

Go to the following location:

C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\Extensions\Xamarin\AndroidSdkManager 

Then run the AndroidSDK Manager Executable.

In bottom right corner, click the gear icon and change the repository to Google.