The latest update keeps saying error

Hi

If you are trying to update your iPhone wirelessly, follow the steps here, including for removing any downloaded update, if required, before trying again to update your iPhone to the latest compatible version of iOS:

• Get help with over-the-air iOS updates - Apple Support
• Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

If you are unable to update wirelessly, consider instead updating via iTunes (and vice versa):

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

Whichever method you choose, be sure to first make a backup of your iPhone (you may wish to choose the option to Encrypt iPhone backup if using iTunes):

How to back up your iPhone, iPad, and iPod touch - Apple Support

Note that, for iTunes to be able to connect with a device running iOS 10 or newer (including updating it), your computer must be running iTunes version 12.5.1 or newer:

Get the latest version of iTunes - Apple Support

What is the error message?