Back when Windows 10 S (now called Windows 10 Pro in S mode, since there's also now a Windows 10 Home flavor) first launched, it was promised that users would be able to upgrade to Windows 10 Pro for free for a limited time. When Windows 10 S became S mode, it was promised that the upgrade to the corresponding full version of the OS would always be free.
But according to various reports, the switch out of S mode isn't working for some users. Normally, it's as simple as opening the Microsoft Store, searching for the option, and installing it, but apparently the page isn't loading, leaving users in S mode, which only allows for them to install apps from the Store.
According to a statement that Microsoft provided to Windows Latest, the company is aware of the problem and it's working on a fix, and there's no estimated timeline for when it will be resolved. According to the Answers forum, you should continue to check for Microsoft Store updates.
It's unclear when this will be fixed. It could be in the next cumulative update, which should be on August 13, or it could be longer, given that this issue might have a relatively low priority. After all, there aren't a lot of devices out there that even ship with S mode, let alone users that haven't switched to full Windows. We were unable to reproduce the bug on a Surface Go or a Surface Laptop.
Update: Microsoft has released a Store update that fixes switching out of S mode, the company released the following statement:
Thank you for your feedback. We have completed our investigation and have published an updated version of the Microsoft Store app which includes a fix for this issue. To install this update: From the taskbar, open Microsoft Store. Select More ... > Downloads and updates > Get updates
You'll need Microsoft Store app version 11906.1001.24.0 or later, as also noted in the comments below and on the support page here.