Status is "Submitted," the user needs to have the permission to "Edit submitted incident report" to delete the attachment. Status is "Closed," you can't delete the attachment directly. First, Unlock the incident, which will change its status to ...
There are a few reasons why an incident can not be edited: If the incident was submitted, only users that have the permission “Edit submitted incident report” can edit. Once the incident status is Closed, it can not be edited.