'System.ValueTuple, Version=0.0.0.0 required for Add-Migration on .NET 4.6.1 Class Library

Add AutoGenerateBindingRedirects and GenerateBindingRedirectsOutputType to your class library csproj

e.g.

<PropertyGroup>
    <TargetFramework>net461</TargetFramework>

    ....

    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
    <GenerateBindingRedirectsOutputType>true</GenerateBindingRedirectsOutputType>
</PropertyGroup>