"You cannot perform this operation on a draft workflow" is totally unhelpful error messages about totally arbitrary rule. Why?
See: https://answers.atlassian.com/questions/19695105
To add insult to injury, the "Help page" linked in the error popup does not contain any information about the problem. You could not care any less, could you?
It's the same answer Christian gave on the question you've linked to. There's a known issue with adding outgoing transitions to status that don't already have them in draft mode.
Yes, it's unhelpful as an error message, and it's a bit of a silly issue (not entirely sure it's a "bug", but it's definitely a problem). I'm not sure if Atlassian plan to deal with it.
You are right, that kind of messages suck.
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