The type namespace name IdentityUser could not be found

This is for Dotnet Core and you will not receive any warning

using Microsoft.Extensions.Identity.Stores

For AspNet Core 1 use NuGet:

Microsoft.AspNetCore.Identity.EntityFrameworkCore

For AspNet Core 2 use NuGet:

Microsoft.AspNetCore.Identity

There is no needs for -

using IdentityUser;
using IdentityDbContext;

Instead you need to add following -

using Microsoft.AspNet.Identity.EntityFramework;

Make sure you have that DLL as a reference as shown below. If it is not available, then you can get that nuget from here.