Update the context

There's not a whole lot going on in the database context yet:

Data/ApplicationDbContext.cs

public class ApplicationDbContext 
             : IdentityDbContext<ApplicationUser>
{
    public ApplicationDbContext(
        DbContextOptions<ApplicationDbContext> options)
        : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        // ...
    }
}

Add a DbSet property to the ApplicationDbContext, right below the constructor:

public ApplicationDbContext(
    DbContextOptions<ApplicationDbContext> options)
    : base(options)
{
}

public DbSet<TodoItem> Items { get; set; }

// ...

A DbSet represents a table or collection in the database. By creating a DbSet<TodoItem> property called Items, you're telling Entity Framework Core that you want to store TodoItem entities in a table called Items.

You've updated the context class, but now there's one small problem: the context and database are now out of sync, because there isn't actually an Items table in the database. (Just updating the code of the context class doesn't change the database itself.)

In order to update the database to reflect the change you just made to the context, you need to create a migration.

If you already have an existing database, search the web for "scaffold-dbcontext existing database" and read Microsoft's documentation on using the Scaffold-DbContext tool to reverse-engineer your database structure into the proper DbContext and model classes automatically.