Why is my iPhone 6+ not updating?

Because the iPhone 6 and 6+ can't run anything higher than 12.4.8.

That's the end of the line for those devices.

If it's running 12.4.8, that model doesn't have enough RAM to run 13 or above.

If it's running a lower version, this depends on what happens when you try to update it.

(184306)