Why won’t my iPhone update

if your details are correct and you have an iPhone 6 Plus.. the latest iOS for you is 12.4.8.

So if you are expecting to be able to upgrade you iPhone to iOS 14, you cannot. For the same reason(s) you were not able to upgrade to iOS 13.

The iPhone 6 can not be updated beyond iOS 12.4.8.

If this is not the issue, post back with details.

Because the iPhone 6 is too old and it lacks the necessary hardware (RAM) for iOS 13 and 14. Please upgrade to a new, current-gen iPhone.