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>