Why can't I update my iPhone 6?

If you're trying to install 13 or above, because that unit doesn't have enough RAM. It isn't upgradable.

If you're trying to install 12.5.5, there will often be a message on the update screen.

If you're trying to install 12.5.4 or earlier, because Apple no longer signs it.

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An iPhone 6 cannot be updated past its current 12.5.5. You would need a 6s or later to install any higher version of iOS.

The non-technical answer........The phone is too old.