Update

The iPhone 6S, released by Apple over seven years ago, cannot be updated beyond iOS 15.7.2.

But I wonder why you require iOS 16 to pair your Apple Watch?

Might you actually have an iPhone 6, which is restricted to iOS 12.5.6?

UPDATE - Niel’s response answers my question. Thanks, Niel.

An iPhone 6s Plus isn't capable of 16 or above and can't be paired with a watch on 9.0 or newer. To update it to 15.7.2, use a computer or the Software Update function in Settings.

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