The most likely reason for being unable to complete the installation is a prerequisite that VS2019 seem to have changed in one of the latter Preview versions - it's called .NET Portable Library targeting pack. Please consider one of the following options :
1. You could install/update the latest Telerik Xamarin VSExtensions from your Extensions tab ( update to the latest version if you already have it, or search for it in the Online VS Marketplace ), in which case the extension would install the missing package and you'd be able to proceed with your installation.
2. You could manually install the missing package from Visual Studio 2019 and proceed with the installation.
Thank you for the feedback regarding the error observed while uninstalling, it has been forwarded to the Reporting team, which will investigate the issue and give an update with their findings.
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