These meeting slots already have meetings scheduled. The system will not allow you to delete these. You can reschedule the meetings, but we strongly advise you to seek the permission of the participants, who may have had good reasons for selecting that timeslot.
In general, we recommend that you do not delete meeting slots after meeting booking has commenced. The system stores a lot of information about meetings, even cancelled ones, and moving meeting slots around can cause it not to work normally in some areas.