Why can’t my iPhone install latest version

Hi there, an iPhone 8 Plus should be able to update to iOS 16 at most, they are not supported for iOS 17 and above.

The iPhone 8 Plus can only updated as recently as iOS 16 since the hardware in that phone is insufficient to run iOS 17 or later versions.

No go if you are asking about installing iOS 17 on the iPhone 8 Plus. You would need an iPhone XR or XS or higher model to handle iOS 17.