When creating the [variables] tag, the following dialog pops out:
After clicking "Yes, I'm sure", the system throws:
So does the [variables] tag exist or not?
I can see a tag synonym from [variable] to [variables] but no questions on either tag. If there is a synonym then the tag existed but it is entirely possible that it was emptied out as it is a meta tag (requires other tags to describe it) and it is not really relevant to this site.
It is possible that the synonym is holding a block on the proper deletion or creation of the tag.
In any case [variables] isn't a particularly useful tag for Super User. Variables are a programming thing and while we allow some "light" programming questions a question that focuses purely on "programming variables" would be off topic here.