Unable to delete or cancel my holiday request

The reason why you can't cancel your holiday request is that your holiday is in the past.
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You're able to delete your approved or pending holiday up until the day it has been booked. Once the holiday is either today or in the past, your manager or an admin can delete this.
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If you're the admin, the same rule applies.
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If your request has been approved by one of your approvers and is still pending approval from your other approver then you will also not be able to delete this request you will need to pending approver to approve the request.