Thank you for reaching out to us on the Microsoft Q&A forum.
We apologize for any inconvenience you may encounter while attempting to perform the use Power Pages design studio to build a secure web page displaying Dataverse data.
Please log out of all open tabs in your current browser and clear the cache files. Subsequently, attempt to log in again using your credentials.
Sign out of your current browser session and attempt to log in again using an incognito window or private browsing mode.
Alternatively, consider switching to a different browser and repeating the login process.
Additionally, we have tried from our end and successfully completed the exercise, as illustrated in the screenshots provided below:
Please reach out to us if you have any other queries.
If the information is helpful, please Accept Answer & Upvote so that it would be helpful to other community members.