Create troubleshooting guide for ApplicationGatewaySkuNotSpecified

Added troubleshooting guide for ApplicationGatewaySkuNotSpecified error, including symptoms, causes, and resolutions.

Author: gitanjali1993

support/azure/application-gateway/error-codes/applicationgatewayskunotspecified.md

@@ -1 +1,48 @@
-TBD
+---
+title: Troubleshoot ApplicationGatewaySkuNotSpecified Error Code 
+description: Learn how to resolve ApplicationGatewaySkuNotSpecified error when you try to create or modify Azure Application Gateway.
+ms.date: 01/22/2026
+editor: giverm
+ms.reviewer: rimayber
+ms.service: azure-application-gateway
+ms.custom: sap:Issues with Create, Update and Delete (CRUD)
+#Customer intent: As an Azure Application Gateway user, I want to troubleshoot an Application Gateway create or modify operation that failed because of ApplicationGatewaySkuNotSpecified error so that I can perform the operation successfully.
+---
+
+# Troubleshoot ApplicationGatewaySkuNotSpecified Error
+
+## Symptom:
+Application Gateway cannot be created or updated because the SKU (size) for the gateway has not been specified resulting in error `ApplicationGatewaySkuNotSpecified`
+
+## Cause:
+The **SKU** (Standard or WAF) for the Application Gateway was not provided during the creation or configuration process.
+
+## Resolution:
+1. **Verify SKU Settings:**
+   - Ensure that the correct SKU is specified when creating or configuring the Application Gateway.
+     - For example, use `Standard_v2` or `WAF_v2` for newer SKU types.
+     - Example configuration:
+       ```json
+       "sku": {
+         "name": "Standard_v2",
+         "tier": "Standard"
+       }
+       ```
+
+2. **Update Application Gateway Configuration:**
+   - If modifying an existing Application Gateway, update the SKU settings using the Azure Portal, CLI, or ARM template.
+
+3. **Check ARM Template/CLI Command:**
+   - If using an ARM template or Azure CLI, ensure that the `sku` section is included correctly.
+     - For CLI:
+       ```bash
+       az network application-gateway create --name <gateway_name> --resource-group <resource_group> --sku Standard_v2 --public-ip-address <public_ip>
+       ```
+
+4. **Re-deploy or Retry Creation:**
+   - After specifying the SKU, retry creating or updating the Application Gateway.
+
+---
+
+### More Information:
+[Application Gateway SKU Types](https://github.com/MicrosoftDocs/azure-docs/blob/cca93efa4de263ed8b6d308183e77a975763ce91/articles/application-gateway/overview-v2.md)