You can only join this beta if you have a factory unlocked device. Go to settings > about phone and look at model name. If it ends in U1, you're eligible. If it ends in U, you're not.
Samsung's US betas are generally open to factory unlocked (U1) devices and T-Mobile locked (U) devices. Other carriers opt to not participate, so their carrier-locked devices are ineligible.
The only One UI 8.5 beta like that is for the S25 series. All other 8.5 betas are only open to factory unlocked devices. It's actually quite unusual for Samsung to open up a beta to so many devices, let alone conduct a mid-cycle beta.
