Disable double click not working

You are misunderstanding what that button actually does. If you read the sentence under it, you will see that it is a way to bring up Apple Pay early.

Turning it off does not mean you don't have to double click at the terminal. If you really want to get rid of that then you need to go to accessibility>touch>assistive touch>confirm with assistive touch - turn that on.

To verify what turning off double click in settings does;

1. Turn double-click off.
2. Go to your Home Screen and attempt to double-click side button. You will notice that the device goes to sleep.
3. Turn double-click on.
4. Go to your Home Screen and double-click the side button. You will notice that Wallet is now showing and ready for the terminal.

When you go to settings>face ID and passcode>Apple Pay -turn that off.

I created a User Tip that has clarified instructions. You can read it here - Double-Click side button usage for Apple … - Apple Community