There are few things to check:
1. Make sure O365 users has license to the product. (Example: exchange mailbox, sharepoint sites)
2. Sufficient license on VB 0365 license.
3. Account connect to 0365 organization should have the following permission listed here:- https://helpcenter.veeam.com/docs/vbo365/guide/vbo_required_permissions.html?ver=30
4. If (1), (2) and (3) is valid, but still receive above error message, then check your deployment.
Install VB 0365 on dedicated VM without sharing with other Veeam product.
Hope this help.
