QR codes cannot be created for live apps until the apps are available to download from the stores. Only when the apps are live will there be a final URL for you to link.
When the apps are live, you can create a QR code with a free tool (as of 31.10.2019), such as: http://onelink.to/
The above tool produces a universal QR code redirects the user to the appropriate App Store, depending on their device. The QR code can be downloaded in either SVG or PNG format.