Hello esther j, thanks for posting this in our community. I completely understand how important it is to have access to the Whiteboard feature, especially when you're conducting online tutoring sessions.
First, make sure you're signed in with a work or school account rather than a personal one like @gmail.com. Also, check that your Microsoft 365 subscription includes access to Whiteboard, as some plans like Teams Essentials may not support it.
If you're part of an organization, you might need to ask your IT administrator to enable Whiteboard. They can do this by going to the Microsoft 365 Admin Center, navigating to Settings, then Org settings, and finally Services. From there, they should ensure that Whiteboard is turned on for your organization.
See: Getting started with Microsoft Whiteboard
Hope this helps!
Kind regards,
John Oli