Hello. Just had the same problem using Safari on imac.
Make sure you're signed out etc. and close window. I had to install chrome (completely new browser for computer) just to get it to work. It did straight away! (Maybe there's some issue with browsers/presonus page saving info etc. for ease of signing in or something?)
Follow the instructions in this link and also see what you qualify for etc.
You click on products in your account - select your qualifying studio one 4 product, then on studio one 4 product page there will be the magical green redeem button.
You initially see a window that only looks like an option to redeem sphere (1st window under 'Redeeming instruction" in link) - you click on that and if you qualified for the Daw upgrade you will see both options as in 2nd window shown. if you only see the sphere option here you didn't qualify for the DAW upgrade. I believe you can get it (studio one 5) somehow as part of the sphere subscription - so free for a year - but then its tied to your sphere subscription. Again not entirely sure about that, do check it out before hand.
Hope this helps.