I am unable to download iOS 15.7 on my iPhone

An iPhone 12 shown in your footer here should be able to run iOS 15.7, and iOS 16.

Usual problems are a lack of the required Wi-Fi or 5G network connection, or using low-powered mode, or having a beta profile present, or having an add-on VPN client app installed, or insufficient free storage.

Are you encountering any error messages or diagnostics?

So that we don’t repeat it, what have you already tried, and what happened?

Hello inez52, What model iPhone do you have? for IOS 15.7 you need iPhone 6s or later.