I had the same problem and just saw the "Offline Activation" button while viewing the info for my Studio One Prime download under "My Account".
1. Login to your PreSonus account, in your browser, using you Google account info.
2. Click "My Account"
3. Find Studio One (version) Prime and click"GET ALL CONTENT".
4. Under your product key click "Offline Activation".
5. Click "Activate Software".
6. Open Studio One Prime. At the activation screen click the "Offline Activation" link at the bottom.
7. Click the copy button to copy your product key.
8. Paste it in the "Enter activation code" box in your browser.
9. Click the windows start button. Right click "Computer" -> "Properties". Copy your computer name.
10. Paste your computer name into the "Enter computer name" box in your browser.
11. Click "Activate".
12. Click "Offline Activation" under your product key again.
13. This time click "Download License".
14. Save the license and remember where you saved it. I suggest the Desktop or your Downloads folder.
15. In the Studio One Prime offline activation screen click the "Browse" button.
16. Navigate to where you saved your license.
17. Click "Activate".
That's the just of it. It may automatically activate once you've selected your license I can't remember. Hope I helped.