In a nutshell: The launch of new devices almost always comes with a host of problems that testing did not reveal. The iPhone 13 has had a few issues come up since launch, the latest being that it does not work with iOS 15's Unlock with Apple Watch feature. Apple is aware of the bug and will address it in an upcoming iOS update.
Last week, new iPhone 13 owners began reporting on Reddit that they were having issues getting the Unlock with Apple Watch to work. Apple started beta testing it on older model iPhones in February and released it in late April. It allows users wearing a face mask to unlock their phone, provided they are wearing their Apple Watch.
However, it appears that Apple's latest iPhones don't play well with the feature. Users trying to use or set up Unlock with Apple Watch get a message saying, "Unable to Communicate with Apple Watch." Others on the Apple Watch subreddit said they had no problems with it on their older phones, so it does not appear to be a problem with a recent update borking the feature.
Apple has acknowledged the issue on its support pages, saying it would fix the problem in an upcoming software update. However, it did not have a timeframe for when the patch would be ready. In the meantime, Apple suggests turning off Unlock with Apple Watch by going to Settings-> Face ID & Passcode.
The Unlock with Apple Watch bug is just another hiccup the has popped up with the iPhone 13 and iOS 15 launch. On Monday, iFixit reported that third-party screen repairs might disable iPhone 13's Face ID functionality. Additionally, researchers revealed three zero-day exploits in iOS 15 just days ago that they claim Apple ignored.