The electronic signature feature is only available in QuickBooks Online Mobile App. You can download and install it through this link: QuickBooks Online Mobile.
To learn more on how to capture signatures using the QBO app, you can check this article: Capture signatures.
In addition, you can also add a digital signature to an invoice by integrating signature apps to QuickBooks Online (QBO).
To check for the compatible apps, you can visit our app center.
Let me know if you have any other questions about adding signatures on invoices. I'll always be here to help!