I initially had this problem with a mac using Safari. I then switched to Chrome, which initially didn't work either. But, it worked after I reset all Chrome settings with Preferences -> Advanced -> Reset settings.
Go to https://my.presonus.com
and login. Click products, Select Studio One 4 Professional. Then you will see a big green "Redeem" button on the screen. Press that, and a window displays giving you the choice of Sphere or upgrade. Select upgrade, and you then go to a screen with your product key and various buttons to download it. Download it, install it, run it, log in with presonus details and away you go.
I suspect a chrome or Safari extension was stopping it working.