EF Core ICollection does not contain a definition for 'Include'

I have a head and a details model with navigation properties between them. I'd like to get head records with details included. This works fine but the detail records I got doesn't contain all properties I need so I tried to add Include().

EF Core says ICollection<RequestDetail> does not contain a definition for 'Include'. I have already tried with List<> navigation types and Microsoft.EntityFrameworkCore and System.Linq are both in my usings.

My models:

public class RequestHead
{
    public string Id { get; set; }
    public DateTime CreateDate { get; set; }
    public int CreateUserId { get; set; }
    [ForeignKey("CreateUserId")]
    public User CreateUser { get; set; }

    public virtual ICollection<RequestDetail> Details { get; set; }
}
public class RequestDetail
{
    [Key]
    public int Id { get; set; }

    public string RequestHeadId { get; set; }        
    [ForeignKey("RequestHeadId")]
    public RequestHead RequestHead { get; set; }

    public DateTime CreateDate { get; set; }
    public int CreateUserId { get; set; }
    [ForeignKey("CreateUserId")]
    public User CreateUser { get; set; }
}

Select:

 var requests = (from r in _ctx.RequestHeads
                select new RequestDTO {
                    AcceptDate = r.AcceptDate,
                    AcceptUser = r.AcceptUser,
                    AcceptUserId = r.AcceptUserId,
                    Details = r.Details != null ? 
 r.Details.Include(x => x.BuyerUser).Select(x => new 
 RequestDetailDTO(x, x.Attachments.ToArray(), x.Product)).ToArray() : null});

Update#1:

This is not a Json.ReferenceLoopHandling problem. It is already ignored in startup.cs and my problem is that I can't use Include() even in my code not just not having the neccessary properties in the json.