If you get the following error when attempting to install the SDK, you might be using an outdated NuGet client. You should consider using an up-to-date NuGet client:
Could not install package 'Intacct.SDK X.X.X'. You are trying to install this package into a project that targets '.NETFramework,Version=v4.6.1', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.