iOS 12 is currently available only as a developer beta, for which you need to pay the yearly membership fee.
Odd, I got a popup on my phone for downloading iOS12 and My development account expired a years ago.
Might be a legacy hangover from having been enrolled in the developer beta scheme with that same device. I just enrolled my spare iPhone 6 into the public beta program and can only get iOS 11.4.1 beta, no iOS 12 beta in sight sadly.