The problem is your entity version is confused with .NetFramework and .NetCore. Your application target framework is Asp.Net Core. So You should install package related with Asp.net Core
In your case 'EntityFramework 6.2.0' is supports by .NETFramework,Version=v4.6.1' not by '.NETCoreApp,Version=v2.0'. So use this below version of entity framework instead of yours.
PM> Install-Package Microsoft.EntityFrameworkCore -Version 2.0.1
3rd party
If Nuget is not installed this command should do it
dotnet add package Microsoft.EntityFrameworkCore