ASP.Net core keeps complaining 'Database operation expected to affect 1 row(s) but actually affected 0 row(s)'

I’m having the above error and don’t know what to do.

You can assume the tables are already set up correctly.
Here is the problem code:

    var setting = _context.Settings.FirstOrDefault();
    ICollection<PD_Datafile> PD_Datafiles = new Collection<PD_Datafile>();

    
    for (int i = 0; i < numfiles; i++)
    {
        DAT_FILE PD_Datafile = ParseFileName(files[i].Name, filepath);
        string databodystr = Encoding.Default.GetString(PD_Datafile.SensorData.DataBody);
        PD_Datafiles.Add(new PD_Datafile() { DataId = i, FullName = files[i].Name, DataBody = databodystr });
    }

    setting.PD_Datafiles = PD_Datafiles;

    ICollection<Alarm> alarms = new Collection<Alarm>();
    alarms.Add(new Alarm() { AlarmId = 0, DateOccurred = 20210120, EquipmentType = "GISPD", FaultType = "PD Happening", Sensor = null, Details = "None" });
    alarms.Add(new Alarm() { AlarmId = 1, DateOccurred = 20210121, EquipmentType = "CBCM", FaultType = "Warning", Sensor = null, Details = "None" });
    alarms.Add(new Alarm() { AlarmId = 2, DateOccurred = 20210122, EquipmentType = "DGA", FaultType = "Danger", Sensor = null, Details = "None" });
    alarms.Add(new Alarm() { AlarmId = 3, DateOccurred = 20210123, EquipmentType = "OLTC", FaultType = "Danger", Sensor = null, Details = "None" });
   
    setting.Alarms = alarms;
    _context.Settings.Update(setting);
    
    _context.SaveChanges();

When the ‘_context’ saves changes, it keeps saying "’Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. "

After some googling, I’ve added this notation above the primary key of each tables as well.

[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]

I think I’m adding the list in the wrong way somehow.

Here are the tables that I’m working with:

namespace OA2.Model
{
    public class Alarm
    {
        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int AlarmId { get; set; }
        public string FaultType { get; set; }
        public int DateOccurred { get; set; }
        public Sensor Sensor { get; set; }
        public string EquipmentType { get; set; }
        public string Details { get; set; }
        [ForeignKey("SettingId")]
        public virtual Setting Setting { get; set; }
    }
}

namespace OA2.Model
{
    public partial class Setting
    {
        
        public Setting()
        {
            PD_Datafiles = new HashSet<PD_Datafile>();
            SCBR_Datafiles = new HashSet<SCBR_Datafile>();
            Alarms = new HashSet<Alarm>();
        }

        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int SettingId { get; set; }
        public int EventAmpTh1 { get; set; }
        public int EventPpsTh1 { get; set; }
        public int EventAmpTh2 { get; set; }
        public int EventPpsTh2 { get; set; }
        public int PulselvlTh { get; set; }
        public long TimePassed { get; set; }
        public string FilePath { get; set; }
        //public virtual PD_Datafile Data { get; set; }
        public virtual ICollection<PD_Datafile> PD_Datafiles { get; set; }
        public virtual ICollection<SCBR_Datafile> SCBR_Datafiles { get; set; }
        public virtual ICollection<Alarm> Alarms { get; set; }

    }
}

Thanks for the help in advance!