Here I am applying the asp.net core identity for user signup

Check Initialization of _SigninManager and _autoservice Objects: Ensure that both _SigninManager and _autoservice objects are properly initialized before using them. Make sure that you have the necessary references and configurations set up for both objects. 

Create a ConnectionString in appsetting.json file under the

 "AllowedHosts": "*",

  "ConnectionStrings": {
    "ConStr": "Data Source=;User Id=sa;Password=*****;Database=;TrustServerCertificate=True;"
  }

Add Service in "Program.cs" class (Asp,Net Core 6.0 and Above):

builder.Services.AddDbContext<your DbContext>(options => options.UseSqlServer(builder.Configuration.GetConnectionString("ConStr")));

Add Service in "Startup.cs" class(Asp.Net Core 5.0) :

In ConfigureServices Method

services.AddDbContext<your DbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("ConStr")))

Exception clearly saying your connection was not initialized.

Please check your connectionstring in appsettings.json file also check program.cs file

It looks like you're encountering an issue related to connection property initialization in ASP.NET. The error you mentioned, "Invalid operation. Connection property is not initialized," suggests that there might be an issue with the way you're using the _SigninManager and _autoservice objects.

Check Initialization of _SigninManager and _autoservice Objects: Ensure that both _SigninManager and _autoservice objects are properly initialized before using them. Make sure that you have the necessary references and configurations set up for both objects. 

var result = _SigninManager.PasswordSignInAsync(username, password, false, lockoutfailure, false).Result;

It's generally recommended to use await instead of .Result to avoid blocking. If _SigninManager.PasswordSignInAsync is an asynchronous method, make sure you are using await correctly and that the containing method is also marked as async.