Give me link software update

1. Is this your corporate iPhone? Then please contact the system administrator of your corporate
2. If it is your personal device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
   1. Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
   2. Or please contact the system administrator of the organisation who provided the profile to you

In such a situation, you might have restrictions enabled in Settings>ScreenTime (preventing software update)

or,

that iPhone is remotely controlled by a company/school/institute who put a configuration profile in Settings>General>Profiles & Device Management.

Regards

Giulio