You are not eligible for an ETA if you are a British citizen.
On first glance, that sounds fairly common sense, as if you're a citizen, why would you need/want one? But there's a wrinkle...
It means that British citizens with dual (or more) nationality must have a UK passport, and must travel into the UK using it, and cannot use their other-nationality passport(s) like they used to be able to do.
Which means paying for a British passport if you didn't have one before.
(There is an alternative, but it's silly money, £589 vs £95 for an adult passport).
And IIRC, the whole thing is because of the new electronic border system that's being introduced or something like that.