Why won't my iPhone XR use the built in QR scanner?
Why won’t my iPhone XR use the built in QR scanner?
[Re-Titled by Moderator]
Sometimes depending on the thing your trying to scan it may not work
See this article: https://support.apple.com/en-us/HT208843
Or use the link above.
You can download multiple QR and barcode scanner apps on the App Store, the one I recommend is QR Reader by TapMedia Ltd