I am Unable to Enter HSN/SAC code In the Invoice.

Nice to have you in the Community space, @balajiandco.

You’ll want to ensure to add an HSN/SAC code in your product and service so it’ll be visible in your invoice. Let me show you how. 

1. Sign in to your QuickBooks Online (QBO) account.
2. Hover to the Gear icon.
3. Select Product and services.
4. If no products or services are added, select Add a product or service.
5. Select product type. 
6. Enter the needed details then add HSN/SAC code. 

For more details, check out this article: How to add HSN and SAC codes to your products and services.

Additionally, ensure to add the HSN/SAC column in your invoice template so you'll be able to view it. To be guided, you can refer to this resource: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Let me also add this link that you can read for reference in paying your taxes in the future: File and pay Tax/GST.

Keep me in the loop if you need further assistance managing your HSN/SAC codes. I’ll be around ready to help you. Keep safe!