Thanks for using Apple Support Communities. From your post, I understand that you're seeing an error after attempting to update your iOS software.
Did the install appear to complete? Which iOS version are you running currently? You can find this under Settings > General > About.
When you updated, was it wirelessly, or using iTunes? If you received the error during or after updating wirelessly, you can use this great article to Get help with over-the-air iOS updates .
If you find the issue persists, and you're still unable to install iOS 12, that's when it may be good to try updating using iTunes. You can find the steps for that here, in the article Update your iPhone, iPad, or iPod touch .
I hope these steps help so you can complete your update.
Take care.