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?