Lock screen

Let's just Force Restart your iPhone:

• Press and quickly release volume up button
• Then immediately press and quickly release the volume down button
• Then immediately press and KEEP HOLDING the side power button (keep holding past the swipe to power off and keep holding until the screen goes black and keep holding until you see the Apple Logo - Then let go)

When your phone comes back up, you'll need to enter your passcode to open it. Then try to customize the lock screen.

Assuming you are using iOS 16, you push harder on the lock screen wallpaper to bring up customization.

What exact model iPhone and what exact iOS version is running on the iPhone?

Settings > General > About > Model and Version

OK.....you're up to date. When the lock screen opens, put your finger in the middle of the screen and push HARD.

Remember that procedure. When something doesn't work as expected, it often fixes things.

And you're welcome.