Use QR Code Reader with deep linking
If you scan a QR code and the QR Code Reader App is not supporting deep linking, the QR Code Reader may not be able to switch to the Facebook app. Try to scan the QR Code with our reference QR Code Reader App.
Check Facebook URL
The other possibility is that you have entered a wrong Facebook URL when you created the Facebook QR Code.
Check if the URL really redirects to your Facebook page.
Check if you are referencing to a Facebook Group
If you want to link to a Facebook Group you need the Facebook Group ID first.
You can find the Group ID it the Source Code of the Group Page. For example go to this group page and press CTRL - u to access the source code. Then search for fb://group/ and you will find a fragment like this where you can copy the Group ID:
<meta property="al:android:url" content="fb://group/691611197690284" />
Now you can use the following URL to create a Facebook QR Code that automagically redirects the user to your group in the Facebook App when installed:
https://www.facebook.com/groups/691611197690284