How to fix iOS 15.8 installation error?

Hi koushik262,

You may want to verify the exact iPhone model (an iPhone 7 can use iOS 15.8), and also provide additional information about the installation problem.

If your iPhone or iPad won't update - Apple Support

iPhone 6

Hi pon113,

With an iPhone 6 (not iPhone 6s), the latest version would be iOS 12.5.7. Upgrading to iOS 13 and higher is not possible for hardware reasons.

An iPhone 6s can use iOS 15.8.

Update iOS 15.8 iPhone 7 Plus