1. Network connection problem: Make sure the device is connected to the Internet correctly, and try to visit other websites or applications to confirm whether the network is working properly. Other websites or applications can be accessed normally, so it is an Apple server-related problem.
2. Apple server problem: Apple will maintain or upgrade its server, which will make it impossible to connect to the service. You can check whether the current server status is normal by visiting Apple official website.
3. Software version problem: Check whether the software version used is the latest version. There is a compatibility problem with the old version of the software, which makes it impossible to connect to the service. Please try to update to the latest version.