How do I use System.Data in a .NET Core RC2 console app (Linux, Debian 8)?

Like poke already annotated in the comment is correct. Specify a version to System.Data.SqlClient makes your restore happy ;)

Why is that? System.Data.SqlClient exists in the http://nuget.org gallery. Not specifying a version ("") is not allowed outside of the boundaries of a project (like a nuget feed package) and specifying solely an star "*" (you should never do that, it allows breaking changes) restore the highest available version. Since there is no stable, star will not find anything (there is some magic with the dashes behind). The RC2 version of that library is the mentioned 4.1.0-rc2-24027 and when you ask with 4.1.0-rc2-* it will take the highest of the RC2 builds (but there is only one). In comparison System.Data.Common has a public release on nuget.org for the Universal Windows Platform and is found for that reason.

The RC3 is the next release and only available on developer feeds from the .NET Core and ASP.NET Core team and not the public nuget feed. You should not play with them.