I don't know the exact reason behind it but I think this is the main reason behind the unavailability and incompatibility of Samsung Pay in Galaxy A71(See the yellow marked area)
I have never seen this message before in A71 updates. But the last update brought this message with it and because of it I think there are some tempering happened with Knox. 🤔🤔🤔
I might be wrong. This is just an assumption.
If you know the exact reason comment below.
Disable biometrics and lock in the lockscreen and disable app lock then update it. In my device samsung pay is still present and I can found it in galaxy store also. After the update me and some other A71 users are facing screen tint and flickering issue randomly, and also sometimes even after restarting the phone some apps are not working (for that I have to reboot the phone several times). Don't know why such bugs are coming from last 2 updates.