Hi Danny,
A) Please confirm on the exercise page, that there is still time remaining on the sandbox. It expires after 1 hour and if you try to do something in the portal after it has expired you will see the behavior you describe. If it has expired, please click the button to Activate a fresh sandbox, wait for it to complete preparing/activating, then navigate to the portal again.
B) In the portal, please click on the gear icon in the upper right corner and make sure that Microsoft Learn Sandbox is showing as Current with green checkmark icon next to it. If it is not current, please click the Switch button next to it to set it as current directory.
I just tested with my account and was able to use sandbox subscription without issue. It could be down for you for some reason, but I wanted to let you know I tested it and at least it is working in my case.
Thanks.
-TP