If you are working with a team-managed project (where validators are not available), you can create an automation rule to reopen any issues that are closed before the checklist is complete:
Reopen Issue If Checklist is Incomplete
Navigate to Project Settings > Automation (or Jira Settings > System > Automation rules).
Click Create rule.
Select the Issue transitioned trigger, and Done as the To status. Click Save.
Click New condition and select Advanced compare condition.
Put
{{issue.Checklist Progress}}
as the First value; does not equal as the Condition; and Empty as the Second value. This will ensure that the rule is only applied to issues that have checklists. Click Save.Click New condition and select Advanced compare condition.
Put
{{issue.Checklist Progress %}}
as the First value; does not equal as the Condition; and100.0
as the Second value. This will check for issues where the checklist is incomplete. Click Save.Select New action and Transition issue. Select the appropriate status and click Save.
Name the rule and click Turn it on.
Tip: You may also want to additional steps to the clear the Resolution and comment as to why the issue was reopened.