A user with Editor permission is unable to delete a milestone. User may see error, "Not authorized to delete: Milestone <MILESTONE_OID>" in Dev Tools Console.
Release : SAAS
Component : AGILE CENTRAL
The milestone is associated with a closed project or work items in a closed project.
Workspace Admin and Subscription Admin are able to delete the milestone. Alternatively, the admins can remove the work items or closed projects associated to the milestone.