ModelState is Invalid AJAX - ASP .NET

I'm doing a project in which at the end of an activity you get a PopUp that when you click on it, it is sent via ajax - api to the database:

Activity Model:

namespace PSCORE.DataBase.Models
{
    public class Activitat
    {
        [Key]
        [MaxLength(100)]
        public string Nom {get;set;}
        //public virtual Regne Regne {get;set;}
        public int Vegades_fet {get;set;} = 0;
        public int Vegades_fet_be {get;set;} = 0;
        public int Vegades_fet_malament {get;set;} = 0;
        public int Progres {get;set;} = 0;
        public float Temps {get;set;} = 0;
    }
}

Api Activity - GurdarInformation:

    namespace PSCORE.Controllers
{
    [Produces("application/json")]
    [Route("api/[controller]")]
    public class GuardarInformacion : Controller
    {
        [HttpGet]
        public ActionResult Index(){
            return Ok();
        }
        private readonly ApplicationDbContext dbContext; 
        public GuardarInformacion()
        {
            dbContext = new ApplicationDbContext();
        }

        [HttpPost]
        public void Guardar([FromBody] ActivitatObject activitat_r)
        {
                Activitat activitat = new Activitat
                {
                    Nom = activitat_r.Nombre
                };

                dbContext.Activitats.Add(activitat);
                dbContext.SaveChanges();        
        }

        public class ActivitatObject {
            public string Nombre {get;set;}
        }
    }
}

Jquery's ajax code:

$(document).ready(function() {
    $(".botonacabar").on("click",function() {

        $.ajax({
            type: "POST",
            contentType: "application/json",
            url: "http://localhost:5000/api/GuardarInformacion",
            data: 
            {
                Nombre: "Prueba"
            },
            dataType: 'json'
        });
    });  
});

I have put a Debug point in the Post of the Api to see if the information arrived or what information arrived, but I got the following error: