Why is my phone unable to update to iOS 14?
Why does my IPhone 6 say that it is up to date when it’s only updated to 12.4.9?
iPhone 6, iOS 12
because your iPhone will never be able to update beyond iOS 12.
Similarly to how you could not download iOS 13 last year.. you also cannot update to iOS 14 this year.
The phone is old enough that it's hardware and RAM are not sufficient for newer iOS versions.
Your iPhone is fully up to date. If you need/want to use newer iOS, you'll need a newer iPhone. (6s/6s Plus or newer)