Hello,
You need to enable QR Codes.

Hi,

To add a QR code to your Odoo invoice PDFs, first enable the QR Codes option in Accounting settings under Customer Payments. Then, configure your bank account journal by filling in the Account Number and Bank details, ensuring your IBAN and BIC are correctly set up. When you create a new invoice, Odoo will automatically populate the Recipient Bank field with your IBAN, and a QR code will be added to the invoice for easy mobile payment.

Reference:-

* https://www.cybrosys.com/blog/how-to-add-a-payment-qr-code-to-invoices-in-odoo-17-accounting

Hope it helps