My iPhone6 can no longer do software update, what’s the problem?

The iPhone 6 has reached the end of the update road if it is running iOS 12.5.5. There is no more road after iOS 12.5.5 for the iPhone 6.

There isn’t one.

the iPhone 6 is to old and hardware limited to run any iOS version past iOS 12.

The iPhone 6 lacks the components to run newer software. It is simply too old! Technology marches on!