C # - EF Core questions to create relational model

I'm starting in Asp Net Core and I have the following question:

I have a Curso template and a Unidade template, a course has multiple units, and that unit can belong to more than one course.

I did the individual Course and Unit model, but my problem is being in the process of creating the relationship model, I was thinking in some way that it would receive a list of courses and a unit, but without success

Below are code examples of my project, while running I'm encountering the error Identity_Insert .

Modelo de curso
public class Curso
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]        
    public long curId { get; set; }

    [Required]
    [StringLength(80)]
    public string curDescricao { get; set; }

    [Required]
    [StringLength(1)]
    public string curStatus { get; set; }

    [StringLength(20)]
    public string curCodExterno { get; set; }

    [StringLength(60)]
    public string curObservacao { get; set; }
}

Modelo de unidade
public class Unidade
{        
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]
    public long uniId { get; set; }
    [Required]
    [StringLength(80)]        
    public string uniDescricao { get; set; }
    [Required]
    [StringLength(1)]
    public string uniStatus { get; set; }
    [StringLength(20)]
    public string uniCodExterno { get; set; }
    public byte[] uniImagem { get; set; }
}

Modelo de CursoUnidade
public class CursoUnidade
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Required]
    [Key]
    public long cuuId { get; set; }

    /*[Required]        
    public long cuuCurId { get; set; }
    [ForeignKey("cuuCurId")]*/        
    public List<Curso> Curso { get; set; }

    /*[Required]
    public long cuuUniId { get; set; }
    [ForeignKey("cuuUniId")]        */
    public Unidade Unidade { get; set; }
}

Serviço de unidade
public void AddTeste(CursoUnidade cursoUnidade)
{
    _contexto.Add(cursoUnidade);
    _contexto.SaveChanges();
}