Dear SamuelMey,
Good day to you!
I hope you are doing well. Thanks for reaching out to our Microsoft Community Forum. We will be happy to help you.
According to your description, someone shared a document with you, and when you try to access the file, you encounter this issue. If my understanding is correct, the sender needs to add you as an external user in their tenant. This process is known as B2B collaboration in Azure Active Directory. The admin must send you an invitation to join their organization as a guest user, and once you accept the invitation, you should be able to access the file.
Additionally, since Microsoft enforces multi-factor authentication (MFA) for all tenants, guest users are also required to complete the MFA process before accessing shared data.
Meanwhile, I would like to know if you are encountering an error when trying to access the file that was shared with you?
I am looking for your response for further assistance.
Thank you for your understanding. Your cooperation and contributions are greatly appreciated.
Kind Regards,
Nahid Sultan