I am trying to set "Field is required Validator" for the Sprint field upon Resolve, but the field is not available in the drop-down list. Is it because it is a locked field? Is there a workaround?
Sprint is a locked field because it's being used by the JIRA Agile add-on for starting sprints (which set the values). Why do you want to set the Sprint field on Resolving a ticket? My points are:
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