I was working on tags (between answers) and when I wanted to suggest a tag synonym I got this message:
What does it mean?
On Stack Overflow, there are tags for code libraries and software development platforms. For instance, there's a jquery tag. In many cases, it is redundant to list tag synonyms for each release of a library update, so the developers put in place validation to prevent people from creating synonyms like jquery1.6 and jquery1.7.
Since this site runs on the same engine, the system thinks you're creating a version-specific tag, and only diamond moderators can create version specific tag synonyms. In short, you're not doing anything wrong, you're just getting caught up in some of the system safeguards.