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Since you update your device to iOS15 and Xamarin.iOS last version support for iOS 14.5, it doesn't include iOS 15 SDK when you deploy app to your device. If you want to use iOS15 features, you need update Xcode, download xamarin.ios-15.0.0.6.pkg and install it to your mac. xamarin.ios-15.0.0.6.pkg isn't a relase version, you also could keep the current version without updating Xcode, but it needs another device to deploy .
Best Regards,
Wenyan Zhang
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