Free users it seems are unable to share boards.
Error that is generated is totally unintelligible.
This functionality needs to updated or removed from the free app.

Request Id: d77c9573-dd6c-4dce-b364-3fd54a008a00
Correlation Id: bd48ff94-f0c8-4933-b27e-cfbf63006733
Timestamp: 2021-01-22T10:37:37Z
Message: AADSTS50020: User account 'ediblecities@outlook.com' from identity provider 'live.com' does not exist in tenant 'Microsoft Services' and cannot access the application '95de633a-083e-42f5-b444-a4295d8e9314'(Microsoft Whiteboard Services) in that tenant. The account needs to be added as an external user in the tenant first. Sign out and sign in again with a different Azure Active Directory user account.