Sorry for this inconvenience.
We could reproduce this issue and the NuGet installation/update problem raised since we have updated the package dependency (based on UWP application). Now we have fixed and refreshed the modified NuGet packages in Syncfusion NuGet server. You can install/update the packages now without any issues.
Please clear the NuGet cache location before install/update the Syncfusion Xamarin NuGet packages. Below knowledge base explained the details about how to clear the cache location form your machine.