Cant update ios 12

Here is a support article with troubleshooting steps for OTA update issues: Get help with over-the-air iOS updates - Apple Support

If those don't resolve the issue, try updating to iOS 12 via iTunes on a computer instead.

Best,

GB

Open iTunes, select the device, and update it. See the Update your device using iTunes section in this support article:

Update your iPhone, iPad, or iPod touch - Apple Support

Best,

GB