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Can you check in the workflow to see if Reopen transition is available. If yes then there might be some condition restricting you to reopen. Check that in Workflow Edit mode --> in diagram mode click on that particular transition and Conditions. You can find more about conditions in the link https://support.atlassian.com/jira-cloud-administration/docs/configure-advanced-issue-workflows/#Advancedworkflowconfiguration-conditions
If there is no transition at all to reopen then You have to create a transition from "On hold" to "Open" status as Reopen.
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