Is there any option on invoice to mention customer Po number ?

Welcome to the Community, Rizrotex.

Yes, there's an option to add the customer's Purchase Order number on your Invoice by adding a Custom Field. Allow me to walk you through the steps:

1. Go to the Gear icon, then select Custom fields.
   
   
2. Click the Add field.
   
3. Enter a name in the Name field.
4. Select All Sales forms and Purchase Order checkbox.
   
5. To show the custom field on printed and delivered forms, turn on Print on form. Otherwise, it'll only appear in QuickBooks Online.
6. Click Save.

For detailed information, refer to this article: Add custom fields to sales forms and purchase orders in QuickBooks Online.

This article will help you customize the appearance and layout of your sales forms to enhance your business communications: Customize invoices, estimates, and sales receipts in QuickBooks Online.

If you have more questions about managing your Invoice, don't hesitate to post them here in the Community. Have a great day!