As the error states these particular Subscribers are a part of Suppression List, you should manually check the list(s) in Email Studio.
In Email Studio go to Subscribers tab and select Suppression Lists from the dropdown menu.
Over there you'll find BU specific lists.
Even if you haven't added anyone yourself, it's likely these Subscribers added themselves if they clicked on the One-Click-Unsubscribe link or - if you have Public Suppression Lists, then they may have gotten to it via Preference Center.
In addition, you can check if there is an Auto-Suppression configured on your Marketing Cloud instance, these have separate lists and custom configuration. You'll find these in the Email Studio > Admin tab.