почему я не могу обновить до 17 ios

Is your phone on this list?

iPhone models compatible with iOS 18 - Apple Support

If your phone is not on this list, then it is not eligible to receive iOS 18, and was not eligible to receive iOS 17. (As far as I know, the iOS 18 list consists of the iOS 17 list, plus all of the iPhone 16-series models.)

iOS 17 hasn't existed for a year now. If you own an iPhone 7/7S/8 or older, it can't support iOS 17 or now higher. An iPhone XS/XR or newer, could have supported iOS 17, when it was still current. The current iOS version is iOS 18.6.2.

What model iPhone do you own?