When you add an organization using the modern app-only authentication method, you are required to provide Azure AD application settings. Please reference following link to create and configure Azure AD Application permissions.
When you add organization using the modern app-only authentication method, the permissions for Azure AD applications that are granted automatically by Veeam Backup for Microsoft 365.
Anyway, if you prefer to use a custom application of your own, make sure to grant all the permissions as below.
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 365 uses such an application to establish a connection to your Microsoft 365 organizations with enabled security defaults and maintain data transfer during backup and restore sessions.