I cannot update my iPhone 6 S plus

Your tag line suggests you have an iPhone 6 Plus, not an iPhone 6s Plus. If you have an iPhone 6 plus, the maximum iOS it is capable of running is iOS 12.5.7. If you have an iPhone 6s plus, it can only run up to iOS 15.8.

In either case, if you are looking for a higher iOS version than the iPhone is capable of supporting, then you'll need to retire you very old (at this point) iPhone and buy a newer, more modern iPhone.

Did you try to connect the iPhone to a PC/iTunes or a MAC and update it from there?

What is the existing version of iOS?