Follow the steps to scaffold the model from existing database in Asp.Net Core:
1. Open your ASP.NET CORE project on Visual Studio.
2. Go to Tools > NuGet Package Manager > Package Manager Console.
3.Run the following command on the console

Install-Package Microsoft.EntityFrameworkCore.SqlServer

4. Again Run,

Install-Package Microsoft.EntityFrameworkCore.Tools

5.Again Run,

Install-Package Microsoft.VisualStudio.Web.CodeGeneration.Design

6.Again Run,

Scaffold-DbContext "Server=localdb)\mssqllocaldb;Database=TestDBName;Trusted_Connection=True;" Microsoft.EntityFrameworkCore.SqlServer -OutputDir Models

Where "TestDBName" is Database name.

If you get an error like 'Scaffold-DbContext' is not known as the name of a cmdlet, then close and reopen Visual Studio.