About assistive touch

You don't have Assistive Touch turned on.

You have tuned on the switch to use "Confirm with Assistive Touch", but you have not actually turned on Assistive Touch. Otherwise, there would be a virtual home button on your screen.

Go to Settings->Accessibility->Touch->Assistive Touch and turn on the switch at the top there.

With that done, you will see a home button in the bottom left corner, when asked to confirm with assistive touch, tap on that virtual home button, then on Device, then on Apple Pay and then tap on Confirm.

There's 2 switches that need to be turned on. "Confirm With Assistive Touch" and the general "Assistive Touch".

Make sure both are turned on. With that, you can tap on the assistive touch virtual button, then on Devices and then on Apple Pay to confirm the purchase.

If you don't want to use Confirm with Assistive Touch, then go to the Assistive Touch screen in Settings, scroll all the way to the bottom, and turn it off.