In case of trouble while installing the package Avalonia, check the NuGet.config:

c:\Users<USER>\AppData\Roaming\NuGet\NuGet.Config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
	<packageSources>
    <add key="nuget.org" value="https://www.nuget.org/api/v2/" />
	</packageSources>
</configuration>