Package Microsoft.EntityFramework.Core 6.0.0 is not compatible with netcoreapp 3.1

As other people also mentioned before, the package you are trying to install is not compatible with the .Net framework used in your project. Open your project in Visual studio and then go to Tools->Nuget Package Manager->Manage Nuget Packages for Solution..., then go to Browse tab and search for "Microsoft.EntityFrameworkCore" when you found the right package click on the dropdownbox close to the version and try different Versions and expand the details to check 'Dependencies' as you may see there version 6.0.x needs at least .Net 6.0 while older versions required .Net standard. Look at the below image for your reference: