Error updating an entity: Conflict with Foreign Key

Hello!

On this action:

[HttpPost, ActionName("Test")]  
    [ValidateAntiForgeryToken]  
    public async Task TestPost(Machine machinetoUpdate)  
    {  
        var machine = _context.Machines.Where(m => m.Id == machinetoUpdate.Id).FirstOrDefault();  
  
        machine.PUnit = machinetoUpdate.PUnit;  
        machine.StoreID = machinetoUpdate.StoreID;  
        machine.Status = machinetoUpdate.Status;  
        _context.SaveChanges();  
        PopulateMachineTypeDropDownListStore();  
  
        return View(await _context.Machines.AsNoTracking().ToListAsync());  
    }  

I'm getting an error while trying to update StoreID.

SqlException: The UPDATE statement conflicted with the FOREIGN KEY constraint "FK_Machine_Store_StoreID". The conflict occurred in database "Application4", table "dbo.Store", column 'StoreID'. The statement has been terminated.

StoreID is a foreign key for the Machine class:

public class Machine  
{  
    public int Id { get; set; }  
    public int TypeID { get; set; }  
    public int SupplierID { get; set; }  
    public int StoreID { get; set; }  
    [StringLength(60)]  
    [Display(Name = "Serial")]  
    public string MchName { get; set; }  
    [DataType(DataType.Date)]  
    [DisplayFormat(DataFormatString ="{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]  
    [Display(Name = "Fecha de Compra")]  
    }  

This is the Store class:

public class Store  
{  
    [Key]  
    public int StoreID { get; set; }  
    public int DistrictID { get; set; }  
    [Required]  
    [StringLength(60)]  
    public string StoreName { get; set; }  
    [StringLength(60)]  
    public string StoreAddress { get; set; }  
  
    public virtual ICollection Machines { get; set; }  
    public virtual District Districts { get; set; }  
}