I'm trying to create a single view application in Xamarin Studio from the multiplatform tab. However the target platform checkboxes as well as the "Next" button are greyed out/disabled. I'm on a Mac.
If you have installed free version of Xamarin long time ago when iOs development was not included for free, the iOs and Android components are missing. Even if you upgrade Xamarin from the "Check for updates", it only upgrades the Xamarin Studio to latest version and doesn't add iOs not Android components. Download software again from Xamarin's web page and install again.
You need to install either Xamarin.iOS or Xamarin.Android SDK's. Then you'll be able to select the checkbox, and Next won't be greyed out.
You can follow further instructions found here. Basically you have to re-run the installer you have, and select Xamarin.iOS and Xamarin.Android when the installer asks you want you want to install.
来源：https://stackoverflow.com/questions/37847640/cant-create-single-view-application-in-xamarin-studio