In Visual Studio v16.2.1, there is no button to include .net core versions in pre-release any more. After upgrading to this version, I am unable to build .net core 3 preview 7 projects.
For .Net Core 3.0 (Preview) use Visual Studio 16.3 (currently Preview channel with Preview 2), this is first version that fully supports .Net Core 3.0 and .Net core 3.0 will ship the same time like VS2019 16.3 final.
NET Core 3.0 Preview
.NET Core 3.0 Preview 8 requires Visual Studio 2019 16.3 Preview 2 or later
来源：https://stackoverflow.com/questions/57477964/visual-studio-2019-16-2-1-no-longer-has-checkbox-to-support-net-core-prerelea