Why this alert is displayed:
Acronis Agent can run only one physical machine backup task at a time. This alert is activated anytime you have a scheduling conflict and two backups tasks are initiated at the same time. In this case, the second backup task is queued until the first one is finished or stopped.

When the alert is deactivated:
Once the second backup is no longer queued, the alert will be deactivated automatically.

How to avoid this alert:
Make sure that your backups are running in the expected time windows and according to their schedule, and avoid scheduling conflicts where possible.