There are two main issues must of us facing with the QR code in Odoo invoice & these issues are
1/ QR code displayed as broken image with the odoo invoice just as in the image blow
Fix : install the compatible version of report lab package for Odoo version
2/ QR code works fine with the invoice preview but it is not displayed with the pdf invoice !
Fix : install the compatible version of wkhtmltopdf package for Odoo version
