Could either be the quota for participants is reached or you have a phone supplied through a network provider Participation is for unlocked phones only currently
I've got s23 ultra I'm in uk and don't see the one ui 6.0 either. Been checking Samsung members all the time, been checking for updates on software updates and nothing 😕
Your phone won't be eligible to participate. Network providers add additional software to optimise network support. So you will probably need to wait until consumer release software is available