My iphone don’t get this update why??

Check all of these carefully:-

·       you are on a good Wi-Fi, (from 15.4 iPhone 7 up, you may be offered cellular update)

·       Plenty of spare storage – 6GB at least.

·       on charge with more than 20%, OR 

·       off charge with more than 50%

·       low power mode turned off. 

·       Fully working cellular network connection via your SIM.

·       Check Settings, General to make sure you have no Beta profile installed.

·       no VPN installed (Check Settings, General, VPN & Device Management)

and see If your iPhone, iPad, or iPod touch won't update - Apple Support