I am unable to update my iPhone 6s

The Related Article is about iOS 17.1, and your tag line says that you have an iPhone 6s running iOS 12.

An iPhone 6s can only update as far as the latest version of iOS 15. It is not capable of updating to iOS 16 or above.

If you are running iOS 12, that makes me wonder whether your iPhone is actually an iPhone 6. An iPhone 6 can only update as far as the latest version of iOS 12. It is not capable of updating to iOS 13 or above.

Do you have an iPhone 6, or do you have an iPhone 6s?

If you have an iPhone 6, the last update for this model was iOS 12.5.7. No more updates after that.

If you have an iPhone 6s, the most recent update for this model is iOS 15.8.2

The iPhone 6s cannot run iOS 16 or iOS 17.