How to test postive and negative tests using moq c# mocking

I am using moq to do some unit testing on my api. However I am relatively new to mocking and am unsure on how to test or mock negative outputs. So for example I can mock creating a user and returning 200 status code, but if i try to create another user that does already exists and return a 400 then i still get 200. If i was not mocking i could do this without issue.

[TestClass]
public class UserControllerTests
{
    private UserController Controller { get; }


    public UserControllerTests()
    {
        var user = new User
        {
            UserName = "testuser@app.com",
            Email = "testuser@app.com",
            EmailConfirmed = true,
            IsEnabled = true,
            StripeCustomerId = Guid.NewGuid().ToString(),
            StripePriceId = Guid.NewGuid().ToString(),
            StripeSubscriptionId = Guid.NewGuid().ToString(),
        };

      
        
        var users = new List<User>
        {
            new()
            {
                UserName = "testuser1@app.com",
                Email = "testuser1@app.com",
                EmailConfirmed = true,
                IsEnabled = true,
                StripeCustomerId = Guid.NewGuid().ToString(),
                StripePriceId = Guid.NewGuid().ToString(),
                StripeSubscriptionId = Guid.NewGuid().ToString(),
            }
        };

        Mock<IUserService> userService = new Mock<IUserService>();

        userService.Setup(x => x.CreateUser(It.IsAny<AddUserDto>())).ReturnsAsync(IdentityResult.Success);
        userService.Setup(x => x.GetUsers(It.IsAny<string>())).ReturnsAsync(users);
        userService.Setup(x => x.FindUser(It.IsAny<string>())).ReturnsAsync(It.IsAny<User>());

        var getUser = new GetUserDto {AllClaims = new List<string>(), Claims = new List<string>(), User = user};

        userService.Setup(x => x.GetUser(It.IsAny<string>())).ReturnsAsync(getUser);
        userService.Setup(x => x.DeleteUser(It.IsAny<DeleteUserDto>())).ReturnsAsync(IdentityResult.Success);
        userService.Setup(x => x.EditUser(It.IsAny<EditUserDto>())).ReturnsAsync(IdentityResult.Success);

        Controller = new UserController(userService.Object);

        var identity = new GenericIdentity("stephen@kaizenappz.com", "test");
        var contextUser = new ClaimsPrincipal(identity);

        var httpContext = new DefaultHttpContext()
        {
            User = contextUser
        };

        var controllerContext = new ControllerContext()
        {
            HttpContext = httpContext,
        };

        Controller = new UserController(userService.Object)
        {
            ControllerContext = controllerContext,
        };
    }


    [TestMethod]
    public async Task WhenANewUserIsCreatedHttpStatusCode200ShouldBeReturned()
    {
        Password password = new Password();
        password.NewPassword = "";
        password.ConfirmPassword = "";

        List<ClaimType> claimTypes = new List<ClaimType>();

        ClaimType claim = new ClaimType {ClaimValue = "admin", Id = 1};

        claimTypes.Add(claim);

        AddUserDto model = new AddUserDto { EmailAddress = "info@kaizenappz.com", Password = password, Firstname = "Info", Surname = "Info", Claims = claimTypes, isEnabled = true };

        var user = await Controller.CreateUser(model);

        if (user is OkObjectResult result)
        {
            var actual = (HttpStatusCode)result?.StatusCode.Value;
            var expected = HttpStatusCode.OK;

            Assert.AreEqual(expected, actual);
        }
    }

}