Why i cant update my phone whats the problem ?

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device. An iPhone 6s Plus will update to iOS 15.7.3

This support article has instructions about updating an iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software:

Update iOS on iPhone - Apple Support

or:

Update iPadOS - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date and won't install anything. Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

To update to the most recent, compatible version of iOS, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information.