You can’t mix .NET Standard only project referencing ServiceStack dependencies with a .NET Framework ServiceStack project, this was never supported. Why did you upgrade to .NET Standard if you’re not going to use .NET Core?
A .NET Framework project uses the .NET Framework dependencies whilst .NET Standard and .NET Core projects use the .NET Standard builds. You can go the other way around have a .NET Core Server sharing .NET Standard only projects with a .NET Framework .NET v4.6.1+ client by getting them to reference the .NET Standard builds in the ServiceStack.Client.Core packages, so they’re all only referencing .NET Standard dependencies.
Not sure what you’re trying to achieve with this hybrid mix of projects but if you want to maintain both .NET Standard and .NET Framework projects you can change your .csproj dependency project to create both .NET Standard 2.0 and .NET Framework .NET v4.6 builds with:
<PropertyGroup>
<TargetFrameworks>netstandard2.0;net46</TargetFrameworks>
</PropertyGroup>
Which will let you reference this project in both ServiceStack .NET Framework and a ServiceStack .NET Core projects.