Why is my iPhone 6+ not updating?

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)

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.