I had same problem yesterday, did everything reboot, clear data cache etc and didn't worked anyways. I live in USA And I noticed that on my Samsung account the region/country wasn't in USA, so I created another Samsung account where the region is USA where I live actually and I registered successfully on beta and downloaded it no issues.
**UPDATE I GOT IT WORKING**
Okay guys. I double checked my region is set to USA and it is. I cleared the cache and data from the Samsung Member app then restarted my phone. I opened the Samsung members app and went through the motions to get the update and it actually worked this time. In my settings it did not say I had a software update but checked it anyway and it all started downloading. Hope this helps. Thank you to those who posted on helped!
If at any point you go through the register and it says "thank you" and won't allow you to register anymore just give it some time and keep checking your software update. If it still doesn't give you the update restart your phone and then check you software update. If it still doesn't show up and you cant register then I'm at a loss at that point.