I can't update iPhone 11

Check all of these carefully especially no. 6

1. you are on a good Wi-Fi, (from 15.4 iPhone 7 up, you may be offered cellular update)  and  that the router is connected to the internet.
2. Plenty of spare storage – 6GB at least.
3. on charge with more than 20%, OR 
4. off charge with more than 50%
5. low power mode turned off. 
6. Check you have no Beta profile installed. (Check Settings, General, VPN & Device Management)
7. no VPN installed (Check Settings, General, VPN & Device Management)

Resources:-

Update iphone

Update your iPhone or iPad - Apple Support

Learn how to update your iPhone via recent Macs here:-->

Update your iPhone, iPad or iPod touch with the Finder – Apple Support (UK)

or older Macs and PC here:-->

Update your iPhone, iPad or iPod touch with iTunes – Apple Support (UK)

KB Article - Iphone will not update

If your iPhone or iPad won't update - Apple Support