Hi,
Before clicking the link in the exercise to open the Azure portal, did you click Activate sandbox, click Review permissions, click Accept permissions (when prompted), etc., until it shows the time remaining on the sandbox? It can sometimes take a couple of minutes for it to finish creating the sandbox subscription.
The sandbox is only valid for 1 hour, so you may need to click Activate sandbox again to create a fresh one. You need to open up the Azure portal immediately after the sandbox has been activated so that you have plenty of time to complete the exercise.
Once you are in the Azure portal, please confirm in upper right corner that it says MICROSOFT LEARN SANDBOX so that you know you are in the correct subscription.
Please try again using the information I have given above and let me know if you have any issues/questions in a comment below.
Thanks.
-TP