UPDATE: This has been confirmed as a "bug", though a pretty obscure one, which is triggered because:
- The original r2-d2 tag was burninated (removed from all questions and auto-purged)
- The tag synonym r2d2 was not purged when the master tag was
- The tag ends in a number -- Stack Exchange has special rules for dealing with tags that "look like" they have version numbers at the end of them.
A dev blew away the synonym; please don't re-create r2-d2
Yes, there's something odd going on with this particular set of tags.
First of all, the answer to your first meta question is a bit misguided. The system does not enforce any such business rule. It's left up to the community editors to make sure that tags are used properly. (In this case, the r2-d2 tag falls into the general category of "character-specific tags", which we usually frown on).
Secondly, the error you are getting is caused by the system mistakenly believing that you are trying to create a "too similar" tag. Normally if you try to create a new tag and there's an existing one that looks very close to it, you won't be allowed. This is to prevent an overload of identical-but-cosmetically-different tags, which is bad.
In this case, r2-d2 already exists; it must have been used on questions in the past. However, it appears to have been removed from those questions, for the reasons laid out in the answer your first question. We don't want people to create tags for specific characters unless there's a really good compelling reason, and when we find such tags we tend to remove them.
Both "exist" as far as the system is concerned, but one is a synonym of the other, and neither are being used by any questions. My guess is, this combination of factors is confusing the software, making it think the tag doesn't exist when it really does.
Note that I've opened a meta-question for this because I'm pretty sure you're found a bug:
Unable to use existing tag because system thinks I'm trying to create it?