See the announcement for ASP.NET Core 3 Preview 4, which explains that this tool is no longer built-in and requires an explicit install:
The dotnet ef tool is no longer part of the .NET Core SDK
This change allows us to ship dotnet ef as a regular .NET CLI tool that can be installed as either a global or local tool. For example, to be able to manage migrations or scaffold a DbContext, install dotnet ef as a global tool typing the following command:
dotnet tool install --global dotnet-ef --version 3.0.0 The reason for the change is explained in the docs:
Why
This change allows us to distribute and update dotnet ef as a regular .NET CLI tool on NuGet, consistent with the fact that the EF Core 3.0 is also always distributed as a NuGet package.
In addition, you might need to add the following NuGet packages to your project: