I am pretty sure that you need a Board Serial Number (SN) from a real Mac. At least about a year ago this was the case. Did you use a SN from a real Mac? If not, then you probably got lucky.

I think there is also the possibility of calling Apple's support hotline and letting them manually activate a randomly generated Serial Number.

See also here: https://www.reddit.com/r/hackintosh/comments/525dsb/getting_...