Solution 1. Hard Restart Your iPhone to Fix Activation Error
Solution 2. Activate iPhone via iTunes to Fix iPhone Activation Error
Solution 3. Restore iPhone to Factory Settings to Fix Activation Error
Reasons Why iPhone Fails Activation after iOS 11 Upgrade
Actually, besides software update, the iPhone activation error may also occur after you restore your iPhone to factory settings. Anyway, if you can't activate your iPhone after iOS 11 update, there are several possible reasons to cause this issue.
1. Busy Apple Server. When a new iOS is available, there should be plenty of Apple device users are trying to update to iOS 11 and this needs Apple Server to deal with them at the same time. It is very easy to cause high traffic and may cause the activation server gets temporarily unavailable or overload when you try to activate your device.
2. iPhone is not recognized. If so, there should be certain local errors preventing your iPhone from contacting the activation server, such as some issue from your carrier, the malfunction or absence of SIM card on your device, or Wi-Fi cannot connect.
3. iTunes is unable to verify your iPhone. There is another possible reason that iTunes can't verify your device due to wrong login details.
Before going further, please make sure your network condition is good, no matter you have connected to Wi-Fi network or cellular data on your device. And in case of data lost, you'd better backup your iPhone data via iTunes or iCloud before trying the following solutions.
See Also: How to Fix iTunes and iPhone Errors?
To get rid of iPhone activation error, you can hard restart your iPhone to give it a fresh restart. It really works on some little problems on device like this. To hard reboot your iPhone, you can follow the instructions as below:
Step 1. Please press and hold the "Power" button and the "Home" button ("Volume Down" button on iPhone 7/7 Plus/8/8 Plus/X or later) at the same time, don't let go until the Apple logo appears on screen.
Step 2. Wait for a while till iPhone screen goes completely black. Then hold the "Power" button to turn on your iPhone again. Now you can check if this problem has been fixed or not.
See Also: How to Solve iPhone Stuck in Boot Loop?
Alternatively, you can use iTunes to activate your iPhone instead. To do so, you can check the following steps.
Step 1. Please launch iTunes on your computer and connect your iPhone to computer with a USB cable.
Step 2. If you have not activated your device, iTunes will prompt you to "Activate iPhone" on the interface.
Step 3. Please select the "Activate iPhone" option and enter your Apple ID and password as prompted, then hit "Continue" button to activate your iPhone.
If you still fail to activate your iPhone, you can try to reset iPhone to factory settings. It is a common and effective way to fix unexpected iOS issues. Importantly, please remember to create a backup of your iPhone contents before restoring iPhone.
Step 1. To reset iPhone, please unlock your iPhone and open Settings app.
Step 2. Then please tap "General" > "Reset" > "Erase All Content and Settings" option.
Step 3. Enter your passcode and tap "Erase iPhone" option to confirm your action and start to restore iPhone to factory default settings.
After restoration, you can try to reactivate your iPhone and see if the iPhone activation error has been fixed.