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It is not possible to change sub-tasks to another type of issue or to change any other type of issue to a sub-task. This is because this due to a Jira specific issue: the transition cannot be done by one spep step only. You have to specify the parrent issue that the sub-task will belong. At the moment this convertion conversion does not work in Issue Navigator Plus.
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In this case you have to (as we suggest) contact jour your Jira administrator, because the absence of some permission settings prevent prevents you from making any kind of modification in the mentioned column. The root of the problem might be that the field is not on the projects' screen or possible possiby is not a mandatory field. Or simply you are not allowed to work with that field.
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