Hello @Mrinal Bakshi ,
Welcome to our Microsoft Q&A platform!
Xamarin.iOS 12 requires Xcode 10.2 (or 10.3) and Xcode 10.2 requires a Mac running macOS 10.14.3 (Mojave) or newer. Since Xamarin.iOS 13.16, the latest features and APIs require Xcode 11.4. And Apple Xcode 11.4 requires a Mac running macOS 10.15.2 (Catalina) or newer which is higher than your mac's. To use the lastest Xamarin.iOS, please update the macOS version on your mac to be applicable with the new version Xcode.
Check the doc: https://learn.microsoft.com/en-us/xamarin/ios/release-notes/14/14.19
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