When a Salesforce Entitlement Management milestone is active on a case and the case status is changed to Closed, users may see the following error when attempting to manually complete the milestone:
"You can't change the completion date on a milestone that's already exited an entitlement process."
This is expected behavior — milestones must be completed before the case is closed.
It's only possible to close or complete milestones for cases that are in an open status. When a case is set to closed status it would be expected for it to exit the entitlement process.
This means that active milestones on cases will need to be closed before the case status is updated or set to closed. If Milestones are not closed before case update they will remain open and the count down stops on the closed case.
As per current application design it's not possible to use Workflow, Process Builder, or Flows to set or update milestone's Completion Date value after case update to closed status.
Administrators will need to utilize Apex to Auto-Complete Case Milestones using a before update trigger when case status is set to closed.
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