Your iPhone cannot use Apple Pay until it has been updated

Check what version of iOS that you have installed by going to Settings > General > Software Update. If your iPhone offers to update to iOS 14.8.1 or iOS 15.1, click Download and Install.

Update your iPhone, iPad, or iPod touch - Apple Support

If your iPhone already has iOS 14.8.1 or iOS 15.1 installed, then you'll need to connect your iPhone to your Mac or PC with iTunes installed.

On a Mac:

1. Open Finder and select your iPhone from the sidebar.
2. Click Check for Update while holding the command key.

On a PC with the latest version of iTunes installed:

1. Open iTunes and find your iPhone.
2. Click Check for Update while holding the control key.

After following these steps, your computer will download the current version of iOS and reinstall iOS on your iPhone while keeping all your files intact.

If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support