To update over the air from the phone all of these requirements must be met:
- You need at least 6 GB of free storage; more is better
 - You must be connected to a Wi-Fi network (and not a personal hotspot) 
 - The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
 - The phone must not be in Low Power Mode
 - The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
 - The phone cannot be in Airplane mode
 - You cannot still have a beta version installed on the phone
 - You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it
 
If you can't meet all of them you can still update using your computer.