Hi Minh,
You can workaround the issue by opening up a new tab and activating sandbox using below link:
Once you have successfully completed the puzzle, accepted permissions, etc., and sandbox is activated, complete the tasks as described in Exercise - Explore the Learn sandbox. Additionally, you may leave the tab open and use the same sandbox for the Exercise - Create an Azure resource later in same module.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP