Currently, in order to use the Telerik's Entity Framework (EF) data source in .NET Core 3.1, .NET 6 and later projects, you should use the EntityFrameworkCoreDataSource product. This uses the EF Core APIs. 

The EntityFrameworkDataSource dll is built under .NET Framework 4.0 and .NET Framework 4.5, and it won't work in .NET 6 projects. Usually, it is recommended to use the EntityFrameworkCoreDataSource along with EF Core, but in some cases, you may need to keep your legacy code and don't want to upgrade from EF 6 to EF Core. 

For these situations, add support for .NET Core 3.1 and later projects (.NET 6, 7, etc.) in the EntityFrameworkDataSource product.