Been trying this for more than 10 times already. Both from iTunes from my laptop and my phone. What should I do? Been doing the usual restart but still that still shows up.
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This happened when there is not enough space.
Before trying to reinstall you need to delete the old update.
To do this go to Settings -> General -> Storage & iCloud Storage -> Manage Storage.
Then look for the update and delete it:
Then try to install the update using iTunes:
Open iTunes and select your device from the left panel
Click on Summary, then click Check for Update
Click Download and Update
When asked, enter your passcode
I had this error with the update to iOS 12 and resolved it by freeing more space on the device, by manually offloading apps or otherwise. I had about 1.5 GB free after download but it only worked after I had about 2 GB free.