Previous versions of Xamarin.iOS (8.9.x and earlier) incorrectly assumed that if the Entitlements.plist file in your project had no keys set, then the app did not need to be signed for use with the iOS Simulator.
This, however, was wrong. The problem is that Entitlements also need to be merged from the selected Provisioning Profile, so simply checking the contents of the Entitlements.plist file was not enough.
Starting with Xamarin.iOS 8.10, if the Entitlements.plist file is set at all for the iPhoneSimulator build configuration, then codesigning is required and thus an iOS code signing certificate is required to be installed in your keychain.
If you do not need your iOS Simulator builds to codesign (because you do not need to test any Entitlement features), then you can go into the iOS Bundle Signing options panel within your Project Options and remove the "Entitlements.plist" value from the Custom Entitlements textbox.