That’s a warning message, not an error message, and this is by design.
Please refer to https://docs.microsoft.com/en-us/nuget/reference/target-frameworks for target framework information.
.NET Standard 2.0 and .NET 4.6.1 have a huge surface area overlap.
For this Visual Studio and NuGet have added the concept of a fallback framework, where when a user tries to install a .NET Standard package in a .NET Framework project, it will allow the installation but warn the user that it’s not 100% compatible and that there may be certain cases in which this does not work.
Related
https://github.com/NuGet/Home/issues/5192
https://github.com/NuGet/Home/wiki/Enable-.NET-Core-2.0-projects-to-work-with-.NET-Framework-4.6.1-compatible-packages
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solved What should I do about unstable packages in Visual Studio 2017, error: was restored using ‘.NETFramework,Version=v4.6.1’ instead of target framework