This code is EntityFramework Code on .NET-8.
expt_table consists of a key and non-key column.
On an empty table, tried to do DbContext.Add of a single record.

namespace expt;

using NUnit.Framework;

using Microsoft.EntityFrameworkCore;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

[Table("expt_table")]
[PrimaryKey(nameof(userName))]
public class DbPageCompleted {

    [Column("user_name")]
    [MaxLength(50)]
    [Required]
    public string? userName {get; set;}

    [Column("last_update_time")]
    [Required]
    public DateTime? lastUpdateTime;
}

public class ExptContext : DbContext {


    public DbSet<DbPageCompleted> pageCompletions { get; set; }

    protected string connectionStr()
    {
        var connStr = Environment.GetEnvironmentVariable("SQLCONNSTR_DbConn");
        if (connStr == null) throw new NoDbConnection();
        return connStr;
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        string? connStr = connectionStr();
        optionsBuilder.UseSqlServer(connStr);
    }

    public static ExptContext dbContext = new ExptContext();
}

[TestFixture]
public class ExptTests {


    DbPageCompleted dbPageCompleted = new DbPageCompleted {
        userName = "[email protected]",
        lastUpdateTime = DateTime.Now.ToUniversalTime()
    };

    [Test]
    public void Expt() {
        ExptContext dbContext = ExptContext.dbContext;
        dbContext.pageCompletions.Add(dbPageCompleted);
        dbContext.SaveChanges();
    }
}

gives an exception

  ----> Microsoft.Data.SqlClient.SqlException : Cannot insert the value NULL into column 'last_update_time', table 'asubio1-db.dbo.expt_table'; column does not allow nulls. INSERT fails.

Have tried a few variants, but have not been able to get past this.
Is there something basic I am missing?