If you are trying out the new Microsoft Whiteboard App (preview) you may be experiencing a login issue.
“your admin has disabled sign in for whiteboard/ please contact your admin to be able to sign in.”
This is because the account you are trying to login with is an Ofifce 365 account and your admins may not have enabled the app within Office 365 yet.
Here is how you enable it:
Within portal.office.com navigate to settings -> services & add-ins
click the whiteboard app
here you can see it’s disabled. Turn it on and you will able to login.
Good luck and enjoy collaborating !