This is because according to a bunch of user reports, it seems that the latest iOS 13.4 update has broken FaceTime for those who own older iPhones and iPad models. It seems that this affects devices that are running on iOS 9, which are basically very old iPhones and iPad models that were launched years ago.
This includes the iPad 2, the third-gen iPad, the iPhone 4S, the first-gen iPad mini, and the fifth-gen iPod touch. According to users, following the update to iOS 13.4, they are unable to connect with these devices via FaceTime anymore. According to an Apple rep who spoke to one of the users experiencing the issue, “We have received reports of this issue and I want to make sure we get this issue resolved as quickly as possible.”
Any other day and this might not have been that big of a problem, but like we said, with people now being kept at home and away from friends and certain family members, tools like FaceTime have become more important than ever. Hopefully Apple will get a fix soon, but in the meantime if you want to avoid these issues, then you might want to skip the update for now.