OK, what am I doing wrong.
I have HK-51 unlocked on a character.
Every other character can see the box on the legacy tab, it says "Unlocked" but has no price.
Nothing happens when I click on it, even if the character has over 1,000,000 credits on them.
You're not looking at the perks of the character, but at the page of the legacy for all characters. Go to the legacy perk page and you can unlock him there.