I created a .NET Standard 2.0 library project using Visual Studio and tried to install Seq as follows.
Install-Package -Id Serilog.Sinks.Seq
The top line of error says
Unable to find package 'Serilog.Sinks.Seq' at source ''.
Full detail is in this screenshot.
Just as a comparison, installing the following Serilog packages on the same project works just fine
Install-Package -Id Serilog
Install-Package -Id Serilog.Enrichers.Thread
What is happening? Is it a .NET Core/Standard issue?
No, the package installs fine into .NET Standard 2.0 class libraries.
The issue here is seems to be where the NuGet client is looking for packages. In the Package Manager Console window, try selecting NuGet.org from the package source drop-down.
(The other packages that are succeeding are probably cached on the machine.)