If you're having trouble logging in to your iCloud account, make sure you're using the correct email address and password. Double-check that the Caps Lock key is not on, as passwords are case-sensitive. If you still can't log in, your account may have been disabled for security reasons, and you'll need to reset your password. In addition, make sure your device is connected to a working Wi-Fi network, and try turning off FaceTime and iMessage temporarily.
Note: Ensure to back up your operating system before updating to avoid any data loss.