Unable to install iOS 15 on my iPhone

Remove any beta profile that might be present, remove any add-on VPN client app, force a restart, connect to Wi-Fi, and try the update again.

How to uninstall iOS or iPadOS beta software - Apple Support

Force restart iPhone - Apple Support

Update your iPhone or iPad - Apple Support

As correctly mentioned above, you should then get offered iOS 15.7.1. You won’t get offered the most recent iOS 16.1.1 on an iPhone 7, as that requires iPhone 8 or newer.

Your iPhone 7 Plus will only upgrade to 15.7. To update, select Settings > General > Software Update. If your phone is eligible for an update, you will be prompted to do so.

If I am not understanding your problem correctly, please provide more detail.