Most iPhones and Androids have the ability to scan QR codes using their camera app, however, this depends on two factors:
The software version of the device
If the user has enabled the QR code reading feature in their camera settings
Android devices overview
All Android devices using software version 9 or above, have the ability to scan a QR code without downloading another app. Version 9 was released on August 6, 2018. Here is how it works (source: @turunen):
Show QR code to the camera
Click on the URL popup
If this is not working for your customers, it's possible they have not enabled the feature. Here's how the feature can be enabled (source: @turunen):
Open Camera and click "More"
Turn on "Google Lens suggestions"
iPhone devices overview
All iPhone devices using software version 11 (iOS 11) or above, have the ability to scan a QR code without downloading another app. iOS 11 was released on September 19, 2017. It works exactly as described above for Android.
If this is not working for your customers, it's possible they have not enabled the feature. Here's how the feature can be enabled (source: imore):
Go to "Camera"
Turn on "Scan QR Codes"
Solution for older versions of the software
If your customers have an older version of the software required to scan a QR code, there are a few options available to them:
Download a free QR code scanning app
Check-in using the unique URL for your location
Get checked in by a member of staff or their group