Why would you need to install EF 6 in regards to using VS2019?
I created a .NET Framework 4.7 WebAPI project, added a 'new item' to project, selected 'Data' and selected 'Data Model' that took me through the wizard of creating a EF model from an existing database, which I would assume it would be some EF 6.x version.
Maybe in your creating of the assumed WebAPI project, you selected a .NET Core framework by mistake, and EF Core doesn't use the 'entity data model' that can be selected from 'Data'.