Hi,
You should be able to work around the issue by first Activating the sandbox (solving puzzles, reviewing/accept permissions, etc.) and then once you see the sign-in box appear on the right open a new browser tab. In this new tab, browse to https://shell.azure.com. If your existing sandbox hasn't expired you can use that instead of activating a new one.
NOTE: I'm including instructions below for using Azure Cloud Shell, even though the exercise you are currently on only uses Azure portal. Completing steps for cloud shell doesn't hurt anything and will be helpful in some later exercises that do require cloud shell.
When prompted, click Bash
Next choose No storage account required, for Subscription select Concierge Subscription, and click Apply.
After several seconds delay while a cloud shell is requested and connected to, you should see $ prompt. At this point you can complete the exercises instructions, open Azure portal in another new browser tab, copy/paste code from exercise tab to cloud shell tab as needed, etc.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP