Can not get web app to write to Azure Storage Blob in Terraform

I tested this on my environment by doing some changes to your code and it worked for me .

1. There is no need to provide these values in app settings i.e. "DIAGNOSTICS_AZUREBLOBCONTAINERSASURL" & "DIAGNOSTICS_AZUREBLOBRETENTIONINDAYS" as when you are enabling logs they will get automatically populated.

2. You should use

     connection_string {
       name  = "StorageAccount"
       type  = "Custom"
       value = azurerm_storage_account.website_log_storage.primary_connection_string
   
     }
   

   Instead of

     connection_string {
       name  = "StorageAccount"
       type  = "Custom"
       value = azurerm_storage_account.website_log_storage.primary_access_key
     }
   

So, after the modifications and some additions the overall code will be like below :

provider "azurerm" {
  features {}
}
provider "random"{}
provider "time" {}
resource "random_string" "myrandom" {
  length  = 6
  number  = false
  upper   = false
  special = false
}
data "azurerm_client_config" "current"{}
data "azurerm_resource_group" "Classroom_In_The_Cloud_Terraform"{
    name="yourresourcegroup"
}
variable "website_name" {
  default = "ansuman-app"
}

// This gets the Azure AD Tenant ID information to deploy for KeyVault. 
resource "azurerm_key_vault" "nscsecrets" {
  name                       = "${var.website_name}-${random_string.myrandom.id}"
  resource_group_name        = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.name
  location                   = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.location
  sku_name                   = "standard"
  tenant_id                  = data.azurerm_client_config.current.tenant_id
  soft_delete_retention_days = 7

}

resource "azurerm_key_vault_access_policy" "client" { // This is for AD Users Logged into Azure to give them the right access when creating resources. 
  key_vault_id        = azurerm_key_vault.nscsecrets.id
  tenant_id           = data.azurerm_client_config.current.tenant_id
  object_id           = data.azurerm_client_config.current.object_id
  secret_permissions  = ["Backup", "Delete", "Get", "List", "Purge", "Recover", "Restore", "Set", ]
  key_permissions     = ["Backup", "Create", "Decrypt", "Delete", "Encrypt", "Get", "Import", "List", "Purge", "Recover", "Restore", "Sign", "UnwrapKey", "Update", "Verify", "WrapKey", ]
  storage_permissions = ["Backup", "Delete", "DeleteSAS", "Get", "GetSAS", "List", "ListSAS", "Purge", "Recover", "RegenerateKey", "Restore", "Set", "SetSAS", "Update", ]
}


resource "azurerm_key_vault_access_policy" "website_accesspolicy" {
  key_vault_id       = azurerm_key_vault.nscsecrets.id
  tenant_id          = azurerm_app_service.website_app.identity[0].tenant_id
  object_id          = azurerm_app_service.website_app.identity[0].principal_id
  secret_permissions = ["get"]
}

resource "azurerm_key_vault_access_policy" "website_logs_storage_accesspolicy" { // This is for the Storage Account for Website Logs. 
  key_vault_id       = azurerm_key_vault.nscsecrets.id
  tenant_id          = data.azurerm_client_config.current.tenant_id
  object_id          = azurerm_storage_account.website_log_storage.identity[0].principal_id
  key_permissions    = ["get", "create", "delete", "list", "restore", "recover", "unwrapkey", "wrapkey", "purge", "encrypt", "decrypt", "sign", "verify", ]
  secret_permissions = ["Backup", "Delete", "Get", "List", "Purge", "Recover", "Restore", "Set", ]
}

resource "azurerm_key_vault_key" "website_logs_key" {
  name         = "website-logs-key"
  key_vault_id = azurerm_key_vault.nscsecrets.id

  key_type = "RSA"
  key_size = 2048
  key_opts = ["decrypt", "encrypt", "sign", "unwrapKey", "verify", "wrapKey", ]

  depends_on = [
    azurerm_key_vault_access_policy.client,
    azurerm_key_vault_access_policy.website_logs_storage_accesspolicy
  ]

}

resource "azurerm_storage_account" "website_log_storage" {
  name                     = "ansumanstorageacc12345"
  resource_group_name      = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.name
  location                 = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.location
  account_tier             = "Standard"
  account_replication_type = "GRS"

  identity {
    type = "SystemAssigned"
  }
}

resource "azurerm_storage_container" "website_logs_container" {
  name                  = "${var.website_name}-cont"
  storage_account_name  = azurerm_storage_account.website_log_storage.name
}
resource "time_rotating" "main" {
  rotation_rfc3339 = null
  rotation_years   = 2

  triggers = {
    end_date = null
    years    = 2
  }
}

data "azurerm_storage_account_blob_container_sas" "website_logs_container_sas" {
  connection_string = azurerm_storage_account.website_log_storage.primary_connection_string
  container_name    = azurerm_storage_container.website_logs_container.name


  start  = timestamp()
  expiry = time_rotating.main.rotation_rfc3339

  permissions {
    read   = true
    add    = true
    create = true
    write  = true
    delete = true
    list   = true
  }

  cache_control       = "max-age=5"
  content_disposition = "inline"
  content_encoding    = "deflate"
  content_language    = "en-US"
  content_type        = "application/json"
}

resource "azurerm_app_service_plan" "websiteappserviceplan" {
  name                = "appserviceplan-dgyn27h2dfoyojc"
  location            = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.location
  resource_group_name = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.name

  sku {
    tier = "Standard"
    size = "B1"
  }
}

resource "azurerm_app_service" "website_app" {
  name                = var.website_name
  location            = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.location
  resource_group_name = data.azurerm_resource_group.Classroom_In_The_Cloud_Terraform.name
  app_service_plan_id = azurerm_app_service_plan.websiteappserviceplan.id

  app_settings = {
    "KEY_VAULT_URL"                        = azurerm_key_vault.nscsecrets.vault_uri
  }

  site_config {
  always_on = true
  dotnet_framework_version = "v5.0"
  app_command_line         = "dotnet EventManagement.Web.dll"
  
  }
  logs{
    detailed_error_messages_enabled = true
    failed_request_tracing_enabled = true
    application_logs {
      azure_blob_storage {
        level="Information"
        sas_url = format("https://${azurerm_storage_account.website_log_storage.name}.blob.core.windows.net/${azurerm_storage_container.website_logs_container.name}%s", data.azurerm_storage_account_blob_container_sas.website_logs_container_sas.sas)
        retention_in_days = 365
      }
    }
    http_logs {
      azure_blob_storage{
        sas_url=format("https://${azurerm_storage_account.website_log_storage.name}.blob.core.windows.net/${azurerm_storage_container.website_logs_container.name}%s", data.azurerm_storage_account_blob_container_sas.website_logs_container_sas.sas)
        retention_in_days = 365
      }
    }
  }

  connection_string {
    name  = "StorageAccount"
    type  = "Custom"
    value = azurerm_storage_account.website_log_storage.primary_connection_string
  }

  identity {
    type = "SystemAssigned"
  }
}

Output:

Note : The above is only for terraform modifications. So after the new app service is created and before you deploy the .net app sample code to the app service you have to do some other modifications to the app code like below.

1. You have add the Microsoft.Extensions.Logging.AzureAppServices package to your dotnet project using the command:

dotnet add package Microsoft.Extensions.Logging.AzureAppServices --version 5.0.10

2. After the above is done you have to paste the below code in your program.cs file of your project.

            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder
                    .UseStartup<Startup>()
                    .ConfigureLogging(logging =>
                    {
                        logging.ClearProviders();
                        logging.AddConsole();
                        logging.AddAzureWebAppDiagnostics();
                    });
                });

3. After the above steps are done you should be able to see the files in the container and if still not reflecting then please go to your

Web app>>App service logs >> click to install the asp.net core site extenstion to enable application logging.

After this you should be able to see the files as below:

Reference for more information on logs:

Configuring Logging in Azure App Services | Blog (ardalis.com)