Unable to update to iOS version 15

If you have iphone 6 Plus then you are stuck at ios12

6S Plus can get ios15

If it really is 6S then what do you see in Settings, General, Software Update?

If you are 2 versions behind and you have a PC or Mac use it to update the phone using USB cable in iTunes or Finder

Why? What happens when you try? Do you get an error? If so, what does it say?

Settings ➜ General ➜ Software Update

If your iPhone 6s is still running iOS 13 as shown in your device line above, then you may need to update through a computer running iTunes (Windows and older Macs), the Devices App(Windows 10 or 11), or the Finder(Newer Macs).

click here ➜ Update your iPhone, iPad, or iPod touch with iTunes - Apple Support