When opting out some prospects from the Outbox page, you're seeing a failure message: Cannot transition to requested state.
Opting out from the Outbox page
All non-delivered mailings
The Opt out feature on the Outbox page only works for successfully delivered mailings. It can only be used with messages that have actually been delivered to a prospect.
You cannot use the Opt out feature for non-delivered mailings: drafted, scheduled, failed, bounced. Since those messages were never delivered to a prospect, the feature won't work for those.