You are using Cloud to Cloud version 2 agent to back up Office 365/ You enabled corresponding quotas and installed the agent. After logging in, you don't see Users in the Backup tab, only Groups and Site collections.
You have not enabled quota for Mailboxes, only for One Drive.
- Open Management Console and navigate to the tenant.
- Under Office 365 seats, mark the Mailboxes quota check-box:
Also, you may need to click the Refresh button in the Cyber Protection console after enabling the quota to detect user mailboxes.