"Unable to find package SwaggerSchema" when installing a package

[norvegec]

>dotnet add package QSwagGenerator for an Asp.Net Core 2.1 xUnit test project results in the following output:

info : Adding PackageReference for package 'QSwagGenerator' into project 'D:\MyTests.csproj'.
log  : Restoring packages for D:\MyTests.csproj...
info :   CACHE https://api.nuget.org/v3-flatcontainer/qswaggenerator/index.json
info :   GET https://api.nuget.org/v3-flatcontainer/qswaggenerator/1.0.0/qswaggenerator.1.0.0.nupkg
info :   OK https://api.nuget.org/v3-flatcontainer/qswaggenerator/1.0.0/qswaggenerator.1.0.0.nupkg 767ms
info :   GET https://api.nuget.org/v3-flatcontainer/swaggerschema/index.json
info :   NotFound https://api.nuget.org/v3-flatcontainer/swaggerschema/index.json 512ms
log  : Installing QSwagGenerator 1.0.0.
error: Unable to find package SwaggerSchema. No packages exist with this id in source(s): Microsoft Visual Studio Offline Packages, nuget.org
error: Package 'QSwagGenerator' is incompatible with 'all' frameworks in project 'D:\MyTests.csproj'.