Why iPhone 6s don’t download Facebook

As I can see, you have an iPhone 6 Plus, and your current phone model only supports up to iOS 12.5.7 (the latest release of iOS 12), If you had an iPhone 6s or an iPhone 6s Plus, then you would have support for up to iOS 15.

In conclusion, your iPhone 6 Plus only supports up to iOS 12.5.7 because it lacks the required hardware for newer and recent versions.

I want to update my software so that I cant download app that needs ios 13.4

iPhone 6 Plus, iOS 12

The iPhone 6 Plus that you have cannot run iOS 13, 14, 15, 16 or 17.

You need an iPhone 7 to run iOS 13, 14 or 15.

You need an iPhone 8 to run iOS 16

You need an iPhone XR or higher model to run iOS 17

The bottom line.....you need a newer phone.

You most likely have an iPhone 6, not a 6S.

A 6S is capable of running iOS 15.8.2.