First solution: Restart your modem/router
Suppose you have errors with iCloud due to network issues such as intermittent connectivity or no connectivity. In that case, the problem is most likely with the network, and that is what you need to tackle first for the iCloud services to work correctly on your iPhone Again. These problems are generally resolved by restarting the wireless modem/router.
Just press and hold the power button on your wireless modem/router until it completely shuts down.
Second solution: check and make sure Wi-Fi is enabled on your iPhone SE.
A Wi-Fi connection is required for iCloud services to work on your device. You will usually see the Wi-Fi icon on your iPhone's status bar if it is enabled. In case you don't see that icon, try checking and turning Wi-Fi on, if necessary.
You can access the Wi-Fi menu using the Control Center. Just swipe up from the bottom edge of the screen and then tap the Wi-Fi icon to enable it.
Alternatively, you can tap the Settings icon on the home screen and then tap Wi-Fi (Settings-> Wi-Fi). Make sure Wi-Fi is enabled. If necessary, tap your preferred Wi-Fi network and enter the password to join.
Third solution: Forget the network and reconnect.
Again, for iCloud issues that are associated with wireless network issues, such as incorrect network passwords or unable to connect to the Wi-Fi network for some reason, you can try forgetting the current network and then reconnecting. This denotes removing the existing network and reconnection to the network to ensure a correct password is used. This is how you do it:
Touch Settings on the home screen.
Tap Wi-Fi.
Touch the wireless network you are currently using.
Tap Forget this network.
Touch Forget to confirm the action.
Other things to consider if your files won't update to iCloud
Make sure you have enough iCloud storage. Buy more, if necessary.
Sign in to iCloud on all your devices and activate iCloud Drive.
Check the internet connection and make sure it is working and stable.
Check and install the latest versions of Pages, Numbers, and Keynote on your device to meet the minimum system requirements with iCloud Drive.
Create a new document and save it to see if it uploads successfully to iCloud. Try other documents if the new document loads.
Update your iPhone SE to the latest version of iOS available. You can test the iOS 11 beta platforms, if necessary, to see if that will fix the problem.