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.
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.