If you're referring to the database first .edmx stuff that EF had then that is not supported in EF Core. EF Core currently only supports class first without using an .edmx.
If you are still on EF 6 then you can install the EF 6 tools in VS and get access to the .edmx designer but it is generally not recommended you use this approach as it is not supported going forward. However there are discussions (search githbu if you're interested) around supporting database first again, it just isn't a high priority.