How can I add digital stamp in invoice? I attached the stamp in invoice and still not working.

I can see how advantageous it is to add digital stamps to invoices, ahad. Let's send this suggestion to our product engineers so they can consider adding this feature in future updates.

For the time being, adding digital stamps to invoices isn't currently an option. With this, I'd recommend sending a request to our product engineers. They continuously seek to fulfill our customer's needs, and your input is vital in deciding which features will be incorporated in upcoming updates. To send feedback, here's how:

1. Go to the Gear icon in the upper right and choose Feedback under Profile. 
2. Type in your recommendation in the Share your feedback field.
3. Click Next to submit.

Meanwhile, you may opt to find other alternatives to help you with your task. To choose the best application for your business, you can visit this page: QuickBooks App Store.

Moreover, to learn what features are available in the new invoice experience, please see this article: See what’s new with estimates and invoices in QuickBooks Online.

Also, I've included this article to guide you in receiving invoice payments in QuickBooks Online (QBO): Record invoice payments in QuickBooks Online.

I'll be here if you have other feature concerns or need further assistance managing invoices in QuickBooks. Drop your comments below, and I'll be sure to help you out.