Adding Organizations with Modern App-Only Authentication
When you add an organization using the modern app-only authentication method, you are required to provide Azure AD application settings. Veeam Backup for Microsoft Office 365 uses such an application to establish a connection to your Office 365 organizations with enabled security defaults and maintain data transfer during backup and restore sessions.
With modern app-only authentication, you cannot use Veeam Backup account; only communications through an Azure AD application is possible.
For limitations in Veeam Backup for Microsoft Office 365 functionality when protecting organizations with modern app-only authentication, see this Veeam KB article.
To add a new Microsoft Office 365 organization, do the following: