Error while validating the service descriptor ‘ServiceType: INewsRepository Lifetime: Singleton ImplementationType: NewsRepository’:

Firstly,you need to change:

services.AddSingleton<INewsRepository, NewsRepository>();

To:

services.AddTransient<INewsRepository, NewsRepository>();

Secondly,you need to inject IMemoryCache instead of MemoryCache in NewsRepository.

Here is a simple demo like below:

1.Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews();
    services.AddSession();
    services.AddTransient<INewsRepository, NewsRepository>();
    services.AddDbContext<BmuContext>(options =>
                options.UseSqlServer(Configuration.GetConnectionString("Connectionstring")));
    services.AddMemoryCache();
}

2.appsettings.json:

"ConnectionStrings": {
    "Connectionstring": "Server=(localdb)\\mssqllocaldb;Database=Bmu;Trusted_Connection=True;MultipleActiveResultSets=true"  
}

3.NewsRepository:

public class NewsRepository : INewsRepository
{
    private readonly BmuContext _context;
    private readonly IMemoryCache _memoryCache;

    public NewsRepository(BmuContext context, IMemoryCache memoryCache)
    {
        _context = context;
    }
    //...
}