My iPhone won’t update ? Why ? V.

Your tag line says that you have an iPhone 6s Plus. You can update an iPhone 6s Plus to the latest release of iOS 15. You cannot update it to iOS 16 or higher. iOS 15 is the end of the line.

About iOS 15 Updates - Apple Support

Please see the user tip below for possible causes and cures.

Updating iOS: common issues and solutions - Apple Community