It was yesterday mid day it was open and likely its full already. I reported last week that itd be out this week for s9 series so I checked twice a day morning then evenings in case. Keep looking in though it may for some reason pop back up again. I have s9+ on Sprint Unlocked
I talked to samsung support and a few people on chat. They said the s9 (usa) has not gotten the beta and probably will not. And it's probably a glitch in system since your getting the banner.
They said it will only allow you in the beta if you have gotten a email from samsung.