AR screen not shown

As noted in the App Store description for TPE, the Augmented Reality (AR) feature requires:

• iOS 11.0 or later
• An iPhone 6S or later, or an iPad Pro or 2017 Model iPad or later, i.e. a device with an A9 processor or later

If you're running an older device, e.g. iPhone 5, 5S, 6, 6 Plus, iPad Air, iPad Mini, the AR function is not available. This is because the AR function is built on Apple's ARKit, which is only available on more recent device models, as explained in this Apple Insider article.