IOS and iPhone 6s Plus

Yes, iPhone 6s can update to iOS 15. But the fact your phone says you are up to date at iOS 12.5.5 is indicative that you likely have iPhone 6, not 6s.

Go to Settings > General > About > Model and double tap it to see the exact model phone you have. If as is suspected, you have iPhone 6, you cannot update to iOS 15 ever.

It's interesting that, if you have a 6s Plus, that it is still running iOS 12.5.5, and that it hasn't been upgraded to iOS 14 yet. Was there some reason you held off, and now decided to jump up to iOS 15?

Please double-check your phone's model name & number (so we can be sure if nothing else): Settings > General > About

You DON'T own an iPhone 6s. You own iPhone 6. And iPhone 6 is not eligible to update to iOS 15. It lacks the hardware necessary. If you want iOS 15, you'll need to buy a newer, more modern iPhone.

You're very welcome!

As lobsterghost1 has mentioned, you have an iPhone 6 Plus, not a 6s Plus. It cannot be upgraded beyond iOS 12.x.

An iPhone 6 Plus cannot update past iOS 12. Do you really have a 6s plus? You need to confirm what phone you really have.