Azure Functions - 403 Forbidden error while calling from inside Azure Portal

I am trying to execute a Timer triggered function via the Azure Portal but I get a 403 Forbidden error:

My networking settings on the Function App:

So it should be available and the appservice itself is running. I have created a new Function App with the default settings on Azure but still get the 403 error. Is there any other setting in Azure that I should set manually to be able to execute the function?

Program.cs

var host = new HostBuilder()
 .ConfigureAppConfiguration((context, config) =>
 {
     var builtConfig = config.Build();

     if (context.HostingEnvironment.IsDevelopment())
     {
         config.AddUserSecrets<Program>();
     }
 })
.ConfigureFunctionsWebApplication()
.ConfigureServices((context, services) =>
{
    var configuration = context.Configuration;
    var connectionString = configuration.GetConnectionString("DefaultConnection") ?? throw new InvalidOperationException("Connection string 'DefaultConnection' not found.");


    services.AddApplicationInsightsTelemetryWorkerService();
    services.ConfigureFunctionsApplicationInsights();
    services.ConfigureFunctionServices();
    services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(connectionString));
    
    services.AddIdentity<ApplicationUser, IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>()
                .AddDefaultTokenProviders();

    services.AddAutoMapper(typeof(Program));
    var mapperConfig = new MapperConfiguration(mc =>
    {
        mc.AddProfile(new ConfigureAutoMapper());
    });
})
.Build();
host.Run();

Function test:

public class FunctionTest
{
    private readonly ILogger _logger;

public FunctionTest(ILoggerFactory loggerFactory)
{
    _logger = loggerFactory.CreateLogger<FunctionTest>();
}

[Function("FunctionTest")]
public void Run([TimerTrigger("0 */5 * * * *")] TimerInfo myTimer)
{
    _logger.LogInformation($"C# Timer trigger function executed at: {DateTime.Now}");
    
    if (myTimer.ScheduleStatus is not null)
    {
        _logger.LogInformation($"Next timer schedule at: {myTimer.ScheduleStatus.Next}");
    }
}

}

I successfully executed the Timer trigger function after updating the below code to retrieve configuration details from local.settings.json, which allowed it to directly access the connection string.

config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
      .AddJsonFile("local.settings.json", optional: true, reloadOnChange: true)
      .AddEnvironmentVariables();
var connectionString = configuration["DefaultConnection"];

Program.cs :

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.EntityFrameworkCore;
using AutoMapper;
using Microsoft.AspNetCore.Identity;
using Microsoft.Azure.Functions.Worker;

namespace FunctionApp16
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var host = new HostBuilder()
                .ConfigureAppConfiguration((context, config) =>
                {
                    config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                          .AddJsonFile("local.settings.json", optional: true, reloadOnChange: true)
                          .AddEnvironmentVariables();

                    if (context.HostingEnvironment.IsDevelopment())
                    {
                        config.AddUserSecrets<Program>();
                    }
                })
                .ConfigureFunctionsWebApplication()
                .ConfigureServices((context, services) =>
                {
                    var configuration = context.Configuration;
                    var connectionString = configuration["DefaultConnection"];
                    if (string.IsNullOrEmpty(connectionString))
                    {
                        throw new InvalidOperationException("Connection string 'DefaultConnection' not found.");
                    }
                    services.AddApplicationInsightsTelemetryWorkerService();
                    services.AddDbContext<ApplicationDbContext>(options =>
                        options.UseSqlServer(connectionString));
                    services.AddIdentity<ApplicationUser, IdentityRole>()
                            .AddEntityFrameworkStores<ApplicationDbContext>()
                            .AddDefaultTokenProviders();
                    var mapperConfig = new MapperConfiguration(mc =>
                    {
                        mc.AddProfile(new ConfigureAutoMapper());
                    });
                    IMapper mapper = mapperConfig.CreateMapper();
                    services.AddSingleton(mapper);
                })
                .ConfigureLogging(logging =>
                {
                    logging.AddConsole();
                })
                .Build();
            host.Run();
        }
    }
}

FunctionTest.cs :

using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;

namespace FunctionApp16
{

    public class FunctionTest
    {
        private readonly ILogger<FunctionTest> _logger;

        public FunctionTest(ILoggerFactory loggerFactory)
        {
            _logger = loggerFactory.CreateLogger<FunctionTest>();
        }

        [Function("FunctionTest")]
        public void Run([TimerTrigger("0 */5 * * * *")] TimerInfo myTimer)
        {
            _logger.LogInformation($"C# Timer trigger function executed at: {DateTime.Now}");

            if (myTimer.ScheduleStatus is not null)
            {
                _logger.LogInformation($"Next timer schedule at: {myTimer.ScheduleStatus.Next}");
            }
        }
    }
}

local.settings.json :

{
  "IsEncrypted": false,
  "Values": {
    "AzureWebJobsStorage": "<StorageConneString>",
    "FUNCTIONS_WORKER_RUNTIME": "dotnet-isolated",
    "DefaultConnection": "<sqlConneString>"
  }
}

I added the DefaultConnection in the Function app > Environment variables > App settings as shown below.

Azure Function app Output :

The Timer trigger function ran successfully as shown below.

Invocations :